Hi, I'm having a problem receiving mails from some sending servers. The logfile shows, that the sending server won't send after the tls session is started:
2026-01-20T14:22:07.126Z,edge01\Internet,08DE372FE331F619,7,10.0.0.4:25,194.42.96.41:35025,*,,"TLS protocol SP_PROT_TLS1_2_SERVER negotiation succeeded using bulk encryption algorithm CALG_AES_256 with strength 256 bits, MAC hash algorithm CALG_SHA_384 with strength 0 bits and key exchange algorithm CALG_ECDH_EPHEM with strength 384 bits" 2026-01-20T14:22:07.141Z,edge01\Internet,08DE372FE331F618,8,10.0.0.4:25,194.42.96.41:46695,-,,Remote(SocketError) 2026-01-20T14:22:07.159Z,edge01\Internet,08DE372FE331F619,8,10.0.0.4:25,194.42.96.41:35025,-,,Remote(SocketError) https://de.ssl-tools.net/mailservers/systema-online.de testing with this tool seems to be different handling when using the server with ecc certificate (edge02) instead of RSA based certificate. Both are Exchange Servers with Edge Role. I contacted the supportdesk of an affected sender (cleverreach) and they send me some telnet logs, but I just don't see any hint if the problem is on my side or if the sending side just handles something wrong. $ telnet edge01.systema-online.de 25 Trying 178.15.145.73...Connected to edge01.systema-online.de.Escape character is '^]'.220 edge01.systema-online.de Microsoft ESMTP MAIL Service ready at Wed, 25 Feb 2026 10:28:08 +0100ehlo crash.crsend.com250-edge01.systema-online.de Hello [80.228.25.228]250-SIZE 20971520250-PIPELINING250-DSN250-ENHANCEDSTATUSCODES250-STARTTLS250-8BITMIME250-BINARYMIME250-CHUNKING250 SMTPUTF8ehlo crash.crsend.com250-edge01.systema-online.de Hello [80.228.25.228]250-SIZE 20971520250-PIPELINING250-DSN250-ENHANCEDSTATUSCODES250-STARTTLS250-8BITMIME250-BINARYMIME250-CHUNKING250 SMTPUTF8500 5.3.3 Unrecognized command 'unknown'ehlo crash.crsend.com500 5.3.3 Unrecognized command 'unknown'ehlo crash.crsend.com250-edge01.systema-online.de Hello [80.228.25.228]250-SIZE 20971520250-PIPELINING250-DSN250-ENHANCEDSTATUSCODES250-STARTTLS250-8BITMIME250-BINARYMIME250-CHUNKING250 SMTPUTF8250-edge01.systema-online.de Hello [80.228.25.228]250-SIZE 20971520250-PIPELINING250-DSN250-ENHANCEDSTATUSCODES250-STARTTLS250-8BITMIME250-BINARYMIME250-CHUNKING250 SMTPUTF8 Anyone has an idea what the problem might be? Regards Norbert
smime.p7s
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