I am having the same issue. It started with the update to 4.4.8.
Debugging from the Test Email gets me this:
2025-04-16 16:34:00 Connection: opening to smtp-relay.gmail.com:587, timeout=300, options=array()
2025-04-16 16:34:00 Connection: opened
2025-04-16 16:34:00 SERVER -> CLIENT: 220 smtp-relay.gmail.com ESMTP 8926c6da1cb9f-4f505cfb9ccsm765956173.9 - gsmtp
2025-04-16 16:34:00 CLIENT -> SERVER: EHLO
www.sunsetonline.org
2025-04-16 16:34:00 SERVER -> CLIENT: 250-smtp-relay.gmail.com at your service, [192.249.120.208]
250-SIZE 157286400
250-8BITMIME
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-CHUNKING
250 SMTPUTF8
2025-04-16 16:34:00 CLIENT -> SERVER: STARTTLS
2025-04-16 16:34:00 SERVER -> CLIENT: 220 2.0.0 Ready to start TLS
2025-04-16 16:34:00 CLIENT -> SERVER: EHLO
www.sunsetonline.org
2025-04-16 16:34:00 SERVER -> CLIENT: 250-smtp-relay.gmail.com at your service, [192.249.120.208]
250-SIZE 157286400
250-8BITMIME
250-AUTH LOGIN PLAIN XOAUTH2 PLAIN-CLIENTTOKEN OAUTHBEARER XOAUTH
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-CHUNKING
250 SMTPUTF8
2025-04-16 16:34:00 CLIENT -> SERVER: AUTH PLAIN
2025-04-16 16:34:01 SERVER -> CLIENT: 334
2025-04-16 16:34:01 CLIENT -> SERVER: [credentials hidden]
2025-04-16 16:34:02 SERVER -> CLIENT: 535-5.7.8 Username and Password not accepted. For more information, go to
535 5.7.8
https://support.google.com/mail/?p=BadCredentials 8926c6da1cb9f-4f505cfb9ccsm765956173.9 - gsmtp
2025-04-16 16:34:02 SMTP ERROR: User & Password command failed: 535-5.7.8 Username and Password not accepted. For more information, go to
535 5.7.8
https://support.google.com/mail/?p=BadCredentials 8926c6da1cb9f-4f505cfb9ccsm765956173.9 - gsmtp
2025-04-16 16:34:02 SMTP Error: Could not authenticate.
I've tried some other settings. Originally it was set for AUTH LOGIN. Same result.