Claws defaults to plaintext for all email protocols. This should be changed to implicit TLS for SMTP (port 465), POP3 (port 995), and IMAP (port 993) and Claws should also warn about plaintext connections.
Historically, port 465 was initially planned for the SMTPS encryption and authentication “wrapper” over SMTP, but it was quickly deprecated (within months, and over 15 years ago) in favor of STARTTLS over SMTP (RFC 3207). Despite that fact, there are probably many servers that support the deprecated protocol wrapper, primarily to support older clients that implemented SMTPS. Unless you need to support such older clients, SMTPS and its use on port 465 should remain nothing more than a historical footnote. The hopelessly confusing and imprecise term, SSL, has often been used to indicate the SMTPS wrapper and TLS to indicate the STARTTLS protocol extension. Defaulting to port 587 with STARTTLS would be the most logical solution. All of the major web email services are implementing that now.
Hey, please see RFC 8314 sec. 3.3. and 7.3.
Evolution, Geary, K9-Mail, ... and multiple Android apps already use port 465 as default. Here is a list of popular mail providers: GMail Outlook.com (465 not supported) Yahoo! Mail GMX, Mail.com, ... Zoho Mail iCloud Mail (465 not supported) AOL MailHushmail FastMail Mailbox.org Mail.Ru Runbox Outlook.com and iCloud Mail don't support SMTP on port 465. I don't think that should be a reason to not use 465 as default...
At the very least, Yahoo, Gmail, AOL, and Fastmail fully support STARTTLS on port 587. I would be surprised to see if the others didn't do so also. Virtually all modern MUAs employ STARTTLS ON PORT 587. I run my own email server and use IMPAP + STARTTLS on port 143, as well as 587. I haven't had any problems. I seriously doubt that Outlook will revert to using a deprecated protocol. Making '465' the default will only result in more service calls. Use 587 and let the casual user who wants to employ 465 do so at their own peril.
The current recommendation from the IETF is: """ To encourage more widespread use of TLS and to also encourage greater consistency regarding how TLS is used, this specification now recommends the use of Implicit TLS for POP, IMAP, SMTP Submission, and all other protocols used between an MUA and an MSP. """ [RFC 8314] This port is not deprecated.