As well had to register to say thank you!
In my case I found the issue was foreign MTA used a slightly lower MTU, which caused a "timeout after DATA" or "timeout after STARTLS" from my MTA (Postfix as well).
I was not able to receive mails from google and some other servers.
I had to decrease my MTU from 1500 to 1472 or less in my case, I found this using "ping"...
I synchronized the MTU (~1400) of my server and its first gateway only (I have some other router layers but with MTU 1500), but don't know if it's really necessary. I'll do some test later...
It works well with this configuration.
Your posts saved a lot of time!