Created attachment 2495 [details] The default connection type provided by CM. When autoconfiguration fails, Claws Mail defaults to the plaintext connection type for email protocols. In "Page index" -> "Advanced", we can see the default ports are 143(IMAP) and 25(SMTP). CM should default to 993, 995, or 465 in all cases.[Ref. RFC 8314].
RFC 8314 is only a "proposed standard", not yet a standard.
You probably also want TLS switched on in the case you describe.