I am experiencing persistent issues configuring a custom-domain email account in Mozilla Thunderbird, while the same account works correctly in Outlook and in webmail. Environment: Email client: Mozilla Thunderbird (latest version) OS: Windows Mail hosting: Antagonist / DirectAdmin (shared hosting) Domain: s23k.com Email address: jean-paul.sablerolle@s23k.com Incoming mail (POP): Server: mail.s23k.com Port: 995 Security: SSL/TLS Authentication: normal password POP connection works without errors. Outgoing mail (SMTP): Server: mail.s23k.com Ports tested: 465 (SSL/TLS), 587 (STARTTLS) Thunderbird fails to send mail with the error: “The requested domain name does not match the server’s certificate.” Even after explicitly adding and permanently accepting the certificate exception, Thunderbird continues to reject the connection. Important observations: Webmail (Roundcube) can send mail successfully. Microsoft Outlook can send mail successfully using the same SMTP server. Thunderbird never completes SMTP authentication because the TLS handshake fails on hostname validation. The certificate presented by the server appears to be issued for a different hostname than mail.s23k.com. As a workaround, I am currently using an external SMTP server (KPN: smtp.kpnmail.nl), which works correctly in Thunderbird. Question: Is Thunderbird intentionally stricter than Outlook regarding TLS hostname validation for SMTP, and is there any supported way to: properly override this hostname mismatch for SMTP, or diagnose whether this behavior is expected and unavoidable with misconfigured server certificates? I would like to confirm whether this is expected Thunderbird behavior or if there is a configuration option I may have missed. I am not trying to weaken security; I want to understand whether Thunderbird allows any safe override in this scenario or if the server configuration is fundamentally incompatible.