I had a problem with error popping up for every computer at our office for sending out estimates or invoices (We have Gmail webmail). It started popping up randomly a few weeks before, but we just pushed the send button again, and it would send. Now though every time it happens.
I got it to work now, here is my fix:
For the Web Mail, select Other instead of Gmail.
Put in the stmp.gmail.com, 587, check box respectively. (Like PBWCPC says)
In Gmail, make sure you have IMAP enabled.
Make sure "Less Secure Apps" is enabled from your google account (This is what fixed it for me)
Then next time you send an email from QB, it will ask you to for your password. It worked for a couple computers in our office.
(Now I am having problem on another computer where it can't access the email server, but that is a separate company/email domain...not sure why though
Shout out to PBWCPC for pointing me to the right direction
