I've been running Thunderbird for years on Gentoo which has version 115.10.1 as the most recent. I don't send much mail so I'm not exactly sure when this problem occurred, but I'm unable to authenticate with my SMTP server. I have not changed any configuration. Some weeks ago a changed my password on the server, but I made the appropriate changes to Thunderbird password manager. All settings, username and passwords are 100% correct as I copy pasted all of them. IMAP on the same server (uses same credentials) works. I have tried deleting and re-adding my accounts which did not help. I have also tried deleting my profile folder. All other mail clients (e.g. claws-mail, kmail, mailspring) with the same SMTP server don't have any problems sending mail. This problem seems to only affect Thunderbird and Thunderbird forks such as Betterbird. I have no firewall and am using SSL authentication with cleartext password on the SMTP server. I get the following output to console on attempted sending of message: console.error: (new Error("Server error 503 Service Unavailable", "resource://services-settings/Utils.sys.mjs", 390)) console.error: mailnews.smtp: "Command failed: 535 Incorrect authentication data; currentAction=_actionAUTHComplete" console.error: mailnews.smtp: "Authentication failed: Incorrect authentication data" console.error: mailnews.smtp: "Authentication failed: Incorrect authentication data" console.error: mailnews.send: "Sending failed; Unable to authenticate to Outgoing server (SMTP) mail.kapsi.fi. Please check the password and verify the 'Authentication method' in 'Account Settings | Outgoing server (SMTP)'., exitCode=2153066805, originalMsgURI="