I have installed 2 Mercury Servers for testing purposes.

I tried tend a batch of 5 messages, using MercuryE on sender and MercuryS on receiver, and SSL on both (self-signed certificates)

Messages 0 1 2 4 arrived

Message 3 failed with "8: Socket read timeout" appearing in the log

That is the best characterization I got

On the Receiver's side, (MercuryS)

The lo is:

08:36:33.401: << 220 OK, begin SSL/TLS negotiation now.<cr><lf>
08:36:33.591: [] SSL/TLS session established
08:36:33.591: [] TLS_AES_256_GCM_SHA384, TLSv1.3, Kx=any, Au=any, Enc=AESGCM(256), Mac=AEAD<lf>
08:36:33.592: [] No peer certificate presented.
08:37:03.857: 8: Socket read timeout
08:37:03.858: >>
08:37:03.872: [] OpenSSL secure session normally terminated.
08:37:03.873: --- Connection closed at 27 Mar 2023, 8:37:03.873. ---
08:37:03.874:

What should I do

Something appears to have been temporarily blocking network communications between sender and receiving server, maybe some anti malware software on the server or on an intermediate device.

(Sorry for not responding earlier - short hospitalization)

It appears that the culprit was AVG, the antivirus software, on the sender's side.

Unticking AVG|Menu|Paramètres|ProtectionBasique|AgentE-Mail|AnalyserLesE-MailsSortants solved the problem

Thanks a lot