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One "weird trick" I discovered with deliverability was that plaintext email from my server was rejected, but HTML email from it was accepted. For anyone with d

by g_p 5y ago

One "weird trick" I discovered with deliverability was that plaintext email from my server was rejected, but HTML email from it was accepted. For anyone with deliverability issues and mail going to spam, it might be worth trying this out (after you have sorted all the usual points like DKIM, SPF, PTR record etc.) - this obviously only helps if your IP is clean. I was surprised though, and only discovered it by accident. I had good deliverability to Google hosted mail, but not to Microsoft 365. I even had issues with deliverability doing 365 to 365, until I sent HTML mail.

dhess 5y ago
As someone who has run his own mail servers since 2000, and still gets his email delivered to people's spam folders (even when I've emailed them in the past, or simply replied to their original message), this is very interesting, as I always enable plaintext outbound email. I'll have to give this a try. Thanks for the tip!
- g_p 5y ago
  Your symptoms sound exactly like the issues I was having - everything was "right" (in theory) - emails were leaving on the right IPv4, that had the correct PTR, etc etc. I just couldn't figure out what was wrong. It was the same issue - even replies to existing threads had the issue. What got me looking into it more closely was that I had no issues delivering to G Suite tenants; it was only issues with 365 tenants. I wonder if there's a non-trivial correlation between people choosing to genuinely self-host their own mail, and those who use plaintext outbound mail by default (and know what that means).
hawski 5y ago
What a world we live in, I always would thought the opposite. Probably it was like that before, but now maybe spammers send more plain text e-mails to appear legit. Never ending game of cat and mouse.
- g_p 5y ago
  My guess (pure speculation) is that more spam is auto-generated via email libraries and similar, and that is producing plaintext mail to try to simplify what it's getting through the filters. On the other hand, most human-generated mail is HTML-based, as it's written in a web-client or desktop/mobile "app client", that is using HTML to let people do bold and put pictures into their email. I was as surprised as you when I figured this out!
upofadown 5y ago
That's odd. I know that Spamassassin actually penalizes HTML and HTML only email. Are you sure there is not some sort of parasitic Bayes filtering happening here?
- g_p 5y ago
  It's very odd. I don't know how Microsoft implements their own server-side spam filtering, but the issue was coming from their filters. No issues with any other host. G Suite accounts received plaintext mail fine. I never found much by way of technical detail on 365 deliverability, beyond the usual DKIM/SPF/DMARC/PTR record guidance, and forms for getting yourself removed from the whitelist. What's especially strange is that this issue applied even when sending mail as a 365 subscriber. Once there was HTML content present in the email, deliverability (to other separate 365 tenants) improved! Very odd...

by g_p 5y ago

dhess 5y ago

As someone who has run his own mail servers since 2000, and still gets his email delivered to people's spam folders (even when I've emailed them in the past, or simply replied to their original message), this is very interesting, as I always enable plaintext outbound email. I'll have to give this a try. Thanks for the tip!

g_p 5y ago
Your symptoms sound exactly like the issues I was having - everything was "right" (in theory) - emails were leaving on the right IPv4, that had the correct PTR, etc etc. I just couldn't figure out what was wrong. It was the same issue - even replies to existing threads had the issue. What got me looking into it more closely was that I had no issues delivering to G Suite tenants; it was only issues with 365 tenants. I wonder if there's a non-trivial correlation between people choosing to genuinely self-host their own mail, and those who use plaintext outbound mail by default (and know what that means).

hawski 5y ago

What a world we live in, I always would thought the opposite. Probably it was like that before, but now maybe spammers send more plain text e-mails to appear legit. Never ending game of cat and mouse.

g_p 5y ago
My guess (pure speculation) is that more spam is auto-generated via email libraries and similar, and that is producing plaintext mail to try to simplify what it's getting through the filters. On the other hand, most human-generated mail is HTML-based, as it's written in a web-client or desktop/mobile "app client", that is using HTML to let people do bold and put pictures into their email. I was as surprised as you when I figured this out!

upofadown 5y ago

That's odd. I know that Spamassassin actually penalizes HTML and HTML only email. Are you sure there is not some sort of parasitic Bayes filtering happening here?

g_p 5y ago
It's very odd. I don't know how Microsoft implements their own server-side spam filtering, but the issue was coming from their filters. No issues with any other host. G Suite accounts received plaintext mail fine. I never found much by way of technical detail on 365 deliverability, beyond the usual DKIM/SPF/DMARC/PTR record guidance, and forms for getting yourself removed from the whitelist. What's especially strange is that this issue applied even when sending mail as a 365 subscriber. Once there was HTML content present in the email, deliverability (to other separate 365 tenants) improved! Very odd...