When eM Client has been running for a while (hours) with me just reading and deleting emails and I try to send an email using my Gmail account, the send fails. Exiting eM Client and starting it up again quickly resolves the problem and sends the email. Easy work around. I am set up with smtp.gmail.com:587 Force usage of SSL/TLS. Is this a Gmail authentication timing issue where eM Client needs to re-authenticate before attempting to send an email after sitting dormant for a long time?