Migrating email service providers can be a large undertaking. We want to help relieve some of that stress of moving to Postmark from Mailgun. This guide will detail some of the differences and similarities between Mailgun and Postmark, as well as give some useful tips for migrating to Postmark from Mailgun.
It includes details on differences between Postmark and Mailgun’s APIs, sending outbound emails, processing inbound emails, UI differences, and webhooks. For quick reference, we have included tables where possible to equate Mailgun functionality and JSON fields to their comparable functionality and fields in Postmark.
Since Postmark does not allow bulk email (including subscription newsletters), there are no list management features available in Postmark, such as handling unsubscribes or adding unsubscribe links to your emails.
Successfully sent messages are stored for 45 days in Postmark (including the content). Bounces and spam complaints are stored indefinitely for troubleshooting purposes.
All Postmark accounts have all features. There are no differences in features between the different plan options.
Postmark does not provide 10,000 free emails each month like Mailgun. We do however offer a 100 emails per month developer plan that is free, so you can try out our service.
Important concepts to learn when moving over to Postmark
There are a couple important concepts to learn when moving over to Postmark: Servers and Sender Signatures, and Verified Domains. A server and a confirmed sender signature/verified domain are required for sending, so it is important to understand what they are used for.
Each Postmark account contains servers. Servers can be thought of as folders you create that group together similar email activity. Each server has its own activity (inbound and outbound), stats, server API token(s), a unique inbound email address, and templates. You can create as many servers as you need, there is no limit.
Some uses of servers are separating your clients’ activity, sending emails for different environments (prod, staging, development), or separating sending for your different domains. When you begin adding more users to your Postmark account, you can also assign them access to specific servers so that they can’t view email activity or change settings across your entire Postmark account.
In Postmark you need to have a confirmed sender signature or verified domain for each email address you want to send from. Sender signatures are individual email addresses that are authorized for sending via a confirmation email sent to that address. Adding and verifying a domain using a DKIM record lets you send from any email address on that domain.
We use sender signatures and verified domains to ensure you own or are authorized to send from the mailboxes you add to your Postmark account. You can have as many signatures and domains as you need, there is not a limit. Sender signatures and domains are associated with your account, not a specific server. This means they can be used for sending across all of your account’s servers.
Tip: You can use any sender signature/verified domain for sending with any Postmark server. It is different from Mailgun where the sending domain and its activity are linked together.
Each Postmark account has an account API token. Every server in an account also has its own server API token(s). Server API tokens are used for server-level actions such as sending email, getting statistics, modifying a template, etc… account API tokens are used for account-level actions such as creating a new server or adding a new domain for sending.
Like Mailgun’s sandbox domain, you can also send test emails to Postmark’s sink email address, email@example.com. Messages sent to this domain will be dropped on the receiving end but you will be able to see the delivery confirmation and message in your Activity. Also see our blog post about best practices when testing with your Postmark account.
Entire domains can be verified for sending using DKIM and SPF records
Similar to how Mailgun allows for adding metadata using the v: prefix, you can add metadata to Postmark messages with both our REST API and SMTP services.
Postmark retains 45 days of full content and message even history, whereas Mailgun only retains 30 days of event history and only retains the full content for rendering for 3 days.
Since Postmark only sends your email from high reputation, fully warmed up IP ranges, there is no chance of your sending rate being throttled or queued, which happens when Mailgun is sending from a newly allocated IP.
Mailgun allows sending messages using a MIME string through their API, which is not supported by Postmark. Emails must be sent through Postmark’s API using the MIME components (To, From, Subject, etc…)
Postmark’s outbound message size limit is 10 MB including attachments, Mailgun’s limit is 25 MB.
Postmark messages can have up to 50 recipients in a single message.
You must use a Server API Token for both your SMTP username and password when using SMTP with Postmark. You can generate as many Server API Tokens as you need.
Postmark does not support scheduling messages to be delivered later. Postmark sends the emails as soon as the request to send is received. If you want to schedule messages to be sent at a later date or time, you would need to do that in your codebase.
Templates are not supported with Postmark’s SMTP service and can only be used with the Postmark API.
Verifying Email Addresses and Domains for Sending #
Similar to Mailgun, domains in Postmark are verified for sending using DKIM and SPF records added to your domain’s DNS. Head over to our help article on verifying a domain for sending with Postmark for more detailed steps.
Postmark also includes an additional option for sending from a single email address that you can confirm using an emailed link. You can add individual addresses as Sender Signatures to your account, which does not require that you add DKIM and SPF records to send with that email address, though we always recommend setting up DKIM and SPF to maximize deliverability.
You can add and manage your sending domains and email addresses from the Sender Signature page in Postmark. Each domain you add has an Authentication page that includes the unique DNS record information for setting up DKIM, SPF, and a custom return-path.
Tip: Postmark does not allow sending from any public domain email addresses, such as Gmail, Yahoo, AOL, Outlook, Live, etc…
Each server you create in Postmark will have its own unique server API token(s). You will need to use a server API token (found in the API Tokens tab of a server in Postmark) for authenticating SMTP sending.
Tip: Port 465 is not supported with Postmark SMTP. If you were using that port with Mailgun, you will need to switch over to using port 25, 2525, or 587 when using Postmark SMTP. We recommend using port 2525 or 587, since port 25 is sometimes blocked by ISPs.
465 for SSL, 25, 587, or 2525 for unencrypted/TLS
25, 587, or 2525
Domain’s SMTP username
Server API Token
Domain’s SMTP password
Server API Token
Unencrypted, SSL, TLS
Plain text (unencrypted), CRAM-MD5, TLS
If you need to whitelist the IPs you connect to Postmark SMTP with, whitelist the following ranges:
When authenticating with the Postmark API for sending emails, you will need to use your server API token in an X-Postmark-Server-Token header. As a reminder, you can get your server API token from the server’s API Tokens tab.
How inbound email processing is implemented is very different between Postmark and Mailgun. If you were using inbound processing with Mailgun and will be using Postmark for processing email once you migrate, this section is an important read.
Postmark's message size limit is 35 MB compared to Mailgun's limit of 25 MB.
Postmark does not require you to take a specific store action (store() in Mailgun) to preserve inbound messages. We will automatically keep them in your Activity and available through the API for 45 days.
With Postmark, you do not have to use a verified domain for inbound domain forwarding, and Postmark does not require that you set up inbound domain forwarding using an MX record in order to use inbound processing. Each server you create in Postmark will come with a unique inbound email address (ex. firstname.lastname@example.org) that you can use to receive emails inbound at your webhook URL. However, if you want to use inbound domain forwarding, and MX record is required.
Postmark receives inbound emails, converts them to JSON, and POSTs them to your inbound webhook URL, rather than processing them using Routes with the data being sent as form data.
Postmark does not have an option to receive inbound messages in their raw MIME form. Inbound messages will always be sent to your inbound webhook URL as well-formatted JSON.
You can use a wildcard in your MX record to have all sub-domains of your domain point to Postmark for inbound processing.
Postmark requires that you enable SMTP (if not enabled already) on your Postmark server to use inbound processing.
Mailgun will retry for 8 hours if there is a failure at the following intervals: 10 minutes, 10 minutes, 15 minutes, 30 minutes, 1 hour, 2 hour and 4 hours. With Postmark, a total of 10 retries will be made, with growing intervals from 1 minute to 6 hours. If all of the retries have failed, your inbound activity page will show the message has a processing error.
Postmark provides you with some additional control on how spam filtering is handled that is not available in Mailgun. In your inbound settings in the Postmark UI (Settings > Inbound) you can set your Spam Assassin threshold, which is the Spam Assassin score that needs to be reached or exceeded to trigger the spam filter (the higher the Spam Assassin score, the more likely the email is spam).
In the inbound spam filtering settings you can also add rules for blocking inbound messages from specific email addresses and domains.
Mailgun and Postmark both allow you to receive notifications as JSON POSTs to URLs you specify when specific events occur using webhooks. Postmark also does not yet have a link tracking webhook, so there is not a way to receive webhook notifications when someone clicks a link in emails you send.
Postmark’s delivery webhook allows you to receive notifications when an email is delivered to a recipient. In Postmark, an email is considered successfully delivered when the destination email server returns a 250 OK response after delivery is attempted.
Postmark includes some additional information for bounce events that is not present in the Mailgun Bounce Event Webhook you should be aware of:
From email address
Subject of the email
Unique identifier for the bounce (used to reactivate a bounced email address using the Postmark Bounce API)
Unique identifier for the sent message (MessageID)
The Postmark server used to send the email
Information on whether the recipient’s email address is deactivated and can be reactivated
Whether a message dump is available. Postmark stores content for 45 days but retains bounce information indefinitely. If the message was sent less than 45 days ago, you can get a full dump of the message content if this parameter’s value is true.
Whether the recipient’s email address can be reactivated
If a message dump is available
Tip: Postmark includes an additional option when setting your bounce webhook URL to include the message content in the JSON sent to your URL. This option lets you receive the full message content when receiving bounce event information.
Like Mailgun, Postmark also has a click tracking webhook available for you to receive notifications when a recipient clicks a link. See below for a mapping of the field names from the Mailgun click tracking webhook to the Postmark click tracking webhook.
When you log into Postmark you are placed in the Servers page, which shows each server you have created. Each server you create has a default transactional message stream for outbound sending and an inbound message stream for processing inbound email. Each server has tabs for Message Streams, Templates, API Tokens, and Settings. Each message stream has tabs for Statistics, Activity, Setup Instructions, and Settings, which are unique for each message stream.
Servers let you separate your outbound and inbound message streams, templates, and credentials based on domains, environments, customers, or any other criteria that helps organize the activity of a given application or website.
The main servers page will show you a list of your servers. You can create as many servers as you need, there is no limit. You can also pin your most frequently accessed servers so they appear at the top of the list. Unused servers will be greyed out, for easy identification of which servers you may want to delete or repurpose.
To view detailed statistics and metrics for a server's message streams, locate the server in the servers list or by using the search field. Click on the bar graph icon and select the server's message stream (inbound or transactional) you are interested in to be taken to its overview page.
To see your inbound and outbound activity in a server, select the message stream you are interested in and click on the Activity tab. This area will show a detailed event view of the stream's events, including sent, delivered, open events, spam complaints, bounces, etc... for the transactional stream and inbound processing events for the inbound stream. Use the search bar to look for emails by subject or email address.
Keep in mind that with Postmark, you do not need to store messages explicitly using a Route to keep them for more than three days. Postmark will automatically store all successfully sent messages for 45 days and all bounces indefinitely.
To see details for a particular event in a stream, click on the event. Some events included in the transactional stream are sent emails (Processed), bounces, spam complaints, clicked links, and opened emails. Events in Activity are color coded to help you tell what occurred at a glance:
Postmark gives you the ability to create and store templates including a variety of pre-built and well-test templates for common scenarios. From the Templates tab you can create, edit, and delete the server’s templates. Each server contains its own templates but you can easily copy templates from one server to another.
Alternatively, if you'd like to build your own batch of templates, we've created and open-sourced MailMason to help you automate the process of creating, testing, and managing your own templates using partials, variables, SASS, and asset management.
The API Tokens tab shows you your server API token(s). Use this tab to generate and delete server API tokens. Server API Tokens are used for outbound sending, SMTP authentication, and making server level API calls.
The Account page is where you can add emergency contacts in case we need to reach you regarding your account and have not heard back from the owner, set up billing notifications, mange your account API tokens, manage your subscription, and other administrative features.
The Users page is where you add and manage users to your account for tasks such as viewing activity for troubleshooting and tracking purposes, managing server settings, creating templates, etc... Use our different roles to effectively manage the security of your account. See our help article on setting permissions for an overview of what options there are and how to control your users’ permissions.
For more Postmark specific insight on how to get started and get the most out of Postmark, make sure to look through our “Getting Started Guide” or visit our support center where you can easily search all of our documentation from a single place. API docs. Guides. Blog posts. Help docs. Labs projects. You name it. We probably have something that can help you out.
Once you've switched to Postmark, you may want to become familiar with our status page and status API. We believe deeply in transparency, and we go a step further than just system availability and share our inbox rates and delivery speeds for the five most popular inbox providers. We also offer all of the data via an API so you can monitor us and set up automation in the event something does go wrong.
You now have a solid understanding of how to transition over the core features you need in an ESP and how to move from Mailgun to Postmark, including how to send outbound and process inbound email, what APIs and webhooks to use for certain functions, and how to view your email activity and statistics. If you have any questions about where to find a setting or how to use a feature in Postmark, get in touch and we can help!