Gmail: Send emails from third-party email address support ends. Starting January 2027.

The "Send As" feature allows you to send emails from any other address you own, such as work aliases or non-Gmail accounts. You can no longer send mail from third-party email accounts in Gmail on the web or in the Gmail mobile apps. Gmail to Gmail aliases and the Google Workspace "Send As" feature aren't affected.

Does this affect the connect-to-Gmail feature in terms of operation?

See announcement: https://support.google.com/mail/answer/17101213?hl=en