Your message file size matters. Some may think it’s great to send a ton of content in one fell swoop, but most ESPs limit the size of the message you can send through their service. That’s likely because they’re sending bulk messages, so they have to keep an eye on the processing power and time it takes to send a lot of very large messages at once.
Even though Postmark permits messages up to 10MB (including headers and attachments), it’s still something I keep my eye on. That's because message size is directly related to email deliverability.
Generally speaking, most messages under 100KB see the same acceptance rates to all mailbox providers. Any larger, however, and we start spotting some alarming trends. I can specifically see at least Apple Mail, Outlook, Google Apps domains, and Yahoo routing these larger messages to spam. Maybe that's because they see too much spam trying to trick their filters with a ton of content? Or perhaps they don't consider the message worth their processing power to filter correctly? Or maybe they think it’s another one of those long emails from your Aunt Bess that you just “sigh” over and immediately delete? #sorrynotsorry
At any rate, if I spot a large message having delivery issues through Postmark, you might get an email from me with some recommendations. I promise it’ll be short! 😉
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