Unable to send email without exiting and reopening

Unable to send mail. I have to exit eM Client and when I reopen it the mail is sent.

No errors on screen? No errors in MENU > OPERATIONS ?
eMC version?

Pro version. I do get a red flag with note - could not send. When exiting, I gat a notice of unsent mail - do I really want to leave. Saying no or anything else doesn’t get it sent. Upon exiting and reopening it gets sent right away. This doesn’t happen on all sends, just after sending others.

Anything in here? Check all tabs.
Again, what numeric version of eMC?

As a test, can you completely disable any anti-virus, firewall, proxy or VPN, then try again.