Since I've started self-hosting my email, I've had constant problems with deliverability to various domains.
On the first iteration of TI, I was sending via my server on port 25 (which had to be opened on special support ticket request to Vultr).
Then, I discovered that emails were bouncing off of Live, Hotmail, and Microsoft 365 Accounts. Solution there was to send a request to the Microsoft Outlook email deliverability department to make a case to un-block my IP (as per this post). That worked.
Most emails were sending successfully at this point, except a few less common domains (Eastlink, Sympatico), and some domains used by larger companies (ones hosted by Postmark, I think). I didn't resolve these.
With a newer iteration of TI, Mailjet is used as an intermediary for email sending, which bypasses the need to open up port 25. This change actually resolved the issues I was having with Eastlink, Sympatico, and Proofpoint hosted emails.
New problems with this change though... now I can't send to Gmail. Apparently Gmail is notorious for deliverability challenges. I'm hopeful that working with Mailjet support, I'll get Gmail receiving my emails...
In the meantime, I use my gmail accounts to send emails to domains that I know will bounce.
At first, I thought this was all a product of my inexperience (it is, to a degree), but I'm starting to sense that I'm not alone in these challenges. Mailjet cites "known issues re: Gmail", and today I discovered a blog post outlining a pretty bleak state-of-the-union for self-hosted email.
More to learn...
Update: The Gmail issue seems to be resolved.