Heho, On Sun, 2023-08-06 at 22:58 +0400, Archange wrote: > isis.lip6.fr
This host has an IPv4 and IPv6 address. If you use the v4 addr. verbatim, the connection fails. If you use the FQDN, you use the v6 addr, the connection works. Works: openssl s_client -connect \[2001:660:3302:283c::2\]:25 -servername isis.lip6.fr -starttls smtp openssl s_client -connect isis.lip6.fr:25 -servername isis.lip6.fr - starttls smtp Does not work: openssl s_client -connect 131.227.60.2:25 -servername isis.lip6.fr - starttls smtp Best guess: The IPv4 address for this host (132.227.60.2) seems to have (i guess) MTU issues (at least for me); From my homebox (mtu <1500 on-path) i cannot connect via v4 on tcp/25, from another host in the same /24 (same routing policy) but 1500mtu on the whole path i can with the s_client line noted as 'does not work' above. Alternatively, they might be doing something else 'somewhat funny'. Not easy to guess, though, without sinking a lot of time, but this really smells like a them-issue to me. Solution: Make lip6.fr fix their stuff. With best regards, Tobias
