Support implicit TLS on port 465

[bugacha]

Summary

When --tls-mode tls is used with SMTP port 465, Mail2Go now dials with implicit TLS (SMTPS) via mail.WithSSL() instead of STARTTLS. Port 465 expects TLS from the first byte, so the previous WithTLSPolicy(mail.TLSMandatory) (which negotiates STARTTLS on a plaintext connection) fails against implicit-TLS-only servers.

Other ports (e.g. 587, 25) are unchanged and keep using STARTTLS via WithTLSPolicy(mail.TLSMandatory).

The underlying go-mail library already supports both modes; this only wires up the port-465 case.

Change

case "tls":
    if smtpPort == 465 {
        clientOptions = append(clientOptions, mail.WithSSL())
    } else {
        clientOptions = append(clientOptions, mail.WithTLSPolicy(mail.TLSMandatory))
    }

Testing

• CGO_ENABLED=0 go build -o mail2go . succeeds.
• Verified against a 465 implicit-TLS endpoint with --tls-mode tls --smtp-port 465.