[exim] TLS error in incoming emails from *.outlook.com

Hello all,

we have a well configured mail server with exi 4.89 on Debian 9, and use a LetsEncrypt certificate for SMTP with STARTTLS on port 25 as well as SMTPS on port 465.

Everything works well, except:

2017-11-27 13:22:55
TLS error on connection from mail-oln040092070016.outbound.protection.outlook.com (EUR03-AM5-obe.outbound.protection.outlook.com) [40.92.70.16] (send): The specified session has been invalidated for some reason.
2017-11-27 13:23:46
1eJILV-0001Gg-K3 TLS error on connection from mail-oln040092071027.outbound.protection.outlook.com (EUR03-DB5-obe.outbound.protection.outlook.com) [40.92.71.27] (recv): Error in the pull function.

It seems only this specific server will fail to connect with TLS with "Error in the pull function". This is the only occasion where we see this message, everything else works fine.

Does someone have an idea how to troubleshoot this, or even how to work around this?

It seems the remote mails server does not back down and tries since days (!) to deliver this message and fails every 5 minutes.

Duh!

Andreas