I am using Thunderbird to access my email server that is running based on Postfix & Dovecot. The server is setup to require StartTLS encryption, and accepts IMAP connections on port 143 and SMTP connections on port 587. The server will reject any login attempts on port 25 for unsecured SMTP. When I setup the account in Thunderbird, I specify these ports and set the connection security to STARTTLS. I am able to retrieve emails without issue using IMAP, but when I attempt to send a message using SMTP, I get the following error: "An error occurred while sending mail: Outgoing server (SMTP) error. The server responded: * OK [CAPABILITY IMAP4rev1 SASL-IR-LOGIN-REFERRALS ID ENABLE IDLE LITERAL + STARTTLS LOGINDISABLED] Dovecot (Ubuntu) ready..." It appears as if Thunderbird is attempting to send the message over port 25 instead of port 587 using StartTLS as specified. I have already tried removing and re-adding the account to Thunderbird. I know my SMTP configuration is okay because I am able to send emails from port 587 without issue from my webmail client (Roundcube).