I use the standard Mail App on my iMac along with Gmail's website (web mail) about equally when sending emails. I have never had any issues until about a month ago. Now I am seeing that certain emails I send are not finding their target, and also, certain emails I reply to are also not making their way. I'm not getting any error messages or undeliverable notices. They are not hiding in other folders, either. Again, this is both thru the Gmail web-based site, and thru the Mail App. And the lost in space messages that i am sending or replying to are going to various providers - Gmail, AOL, Yahoo, Ezweb, Openmailbox.org, etc. So it's not just one "bad" address. Looking in the settings thru Mail App, Preferences - it looks/acts a little weird to me from previous memory. Mail App < Preferences < Accounts < Account Information < Outgoing Mail Server (SMTP) < Edit: 1) TLS Certificate is 'None' : Is this ok? Other choice is really long com.apple.idms.appleid........ 2) Advanced : Port 587 / Use SSL (checked) 3) Authentication : It keeps fluctuating between 'None' and External (TLS Cert) - but in years past, 'Password' was fine. anytime I change this and save it, it won't hold the change/save. Mail App < Preferences < Accounts < Advanced: 1) Port 993 / Use SSL (checked) 2) Authentication : It keeps at External (TLS Cert) - again, I'm used to it being listed as 'Password', but trying to change and save, it reverts back to this. Mail App Connection Doctor: IMAP and SMTP both list as 'Connection and Login to Server Succeeded.' But only the SMTP has the little green dot next to it - IMAP does not.