[BlueOnyx:24729] Deferred: Connection timed out with mail.domain.tld — forgot signature!

[Meaulnes Legler @ MailList]

hello list

I don't get it: recently, e-mails sent from one virtual server to anotherone on the same 5209R box are rejected with the following:
_________________________________________________________________________
     **********************************************
     **      THIS IS A WARNING MESSAGE ONLY      **
     **  YOU DO NOT NEED TO RESEND YOUR MESSAGE  **
     **********************************************

The original message was received at Wed, 20 Jan 2021 12:40:56 +0100
from 238.132.61.188.dynamic.wline.res.cust.swisscom.ch [188.61.132.238]

    ----- Transcript of session follows -----
<user at domain.tld>... Deferred: Connection timed out with mail.domain.tld.
Warning: message still undelivered after 4 hours
Will keep trying until message is 5 days old
_________________________________________________________________________

• I couldn't find out from which senders domains it happens, fromgmail.com senders for instance it doesn't happen
• sendmail and dovecot are running without errors
• the senders aren't listed in the APF blacklist.
• after 5 days nothing is sent

I rebooted the servers but it didn't help, the e-mails sent from and to domains on the same 5209R box are deferred and not sent. If I send them from my gmail account, they are accepted.

any ideas what's going on?

thank you and best regards

で⊃ Meaulnes Legler
Zurich, Switzerland
+41¦0 44 260-1660