Apple Mail fails to send emails on my Mac unless I edit after hitting send

MacOS 15.5 on MacBook Air M2

Using the Apple Mail app sending email is failing when clicking the 'send' arrow. The message just sits there unless I start editing it further, then it goes immediately. For now I click 'send' and then just add a newline at the end of my message so that it completes the send.

Happens on Apple Mail when sending with properly-configured GMail (Apple config) or FastMail (IMAP) services (I do not use iCloud mail), whether or not the message includes an attachment.

[Re-Titled by Moderator]

Original Title: macos 15.5 mail fails to send