To add a TXT record for DKIM and a CNAME record for a custom Return-Path to your DigitalOcean hosted domain, follow these steps.
Obtain the DKIM and Return-Path records from Postmark
The first step is to obtain the DKIM TXT record from your DNS Settings page for the domain you are adding DKIM for. Log into Postmark and navigate to the Sender Signatures page. Click 'DNS Settings' for the Sender Signature or Domain you are adding DKIM and Return-Path records for.
You will then see entries for both DKIM and Return-Path:
These records will be used once you login to DigitalOcean, so keep the window or tab open in your browser.
Adding Records to DigitalOcean
Log in to DigitalOcean and click Networking.
Once you are in the Networking area, click Domains on the left.
Click on the blue magnifying glass icon for the domain you are adding the records for. You will then see an option to add a record to your DNS.
To add your DKIM record, click TXT for the type and for Name, enter the Hostname shown for the DKIM record in your DNS Settings page in Postmark. Populate the Text field with the DKIM record's value from the DNS Settings page in Postmark.
IMPORTANT: The text added for each record must be surrounded in quotes to be accepted.
Next add a CNAME record in a similar fashion. Be sure to change the record type to CNAME and then for the Name enter in the Hostname from your DNS Settings page for the Return-Path. For the value, enter pm.mtasv.net.
Once both of these records have been added you will see them in your list of DNS records in Digital Ocean. For DKIM, Postmark will check your DNS records every five minutes to see if it is valid. Once it is identified, the DKIM record will be verified automatically. You can also click the Verify button to manually check the DKIM record.
If everything is propagated and working, you will see a green checkmark in your Sender Signatures page showing that the domain has been verified. You can now also send from any email address on that domain, not just the original Sender Signature that was added.