Why does postfix regularly have "timed out while sending message body"?

Question

postfix

Colleagues, good afternoon.

Help, please, to understand what could be the matter. I've already broken my head.

In the postfix logs I regularly see the lines

2020-10-08T11:44:30.173620+02:00 mail postfix/error[9395]: 06964341304: to=, relay=none, delay=827, delays=560/267/0/0, dsn=4.4.2, status=deferred (delivery temporarily suspended: conversation with alt1.gmail-smtp-in.l.google.com[108.177.97.27] timed out while sending message body )

Previously, this was only with incoming letters to me. We talked with the administrator of one sending server, ran tcpdump on both sides, it was clear from the dump that my server sends “repeat fragment X” packets, but its server does not receive this packet and it (the sending server) sends further packets no matter how what never happened. And after a while the connection is broken and the entry in the log is as above. Tried to reduce mtu to 1000, did not help. He and I do not have fragmentation packet blocking on the firewall. But there were up to 80% packet loss on the route from me to him and vice versa ("mtr --tcp -n IP").
Today mail from me "takes a long time". Users say that they sent a letter at 8, the recipient received it in half an hour. In the logs, the timeouts are as above.

Yesterday, users of another server complained that the mail takes a long time (1-2 hours, sometimes 1-2 days). Checked, found out that on the server gmail, yahoo, outlook (dozens of delivery attempts) and several Russian state. bodies the same "timed out while sending message body". Part of routes without packet loss, part with loss.

These timeouts only occur with emails with attachments larger than ~2 MB. If the letter is without an attachment, then the mail goes without problems.

It all started in the spring and appears periodically, sometimes mail goes normally with attachments, then again timeouts. I used to notice this only with mail to me, yesterday I noticed it with mail from me. This happens on two different servers, different OS, different versions and postfix configs, different domain names.

What can be wrong? Do you have such problems?