I think I figured it out! This is what worked for me (using Calibre and gmail):
In Calibre go to "
Preferences" > "
Sharing books by email" :
In the "
Send email from box:" make sure your entire gmail address is appearing
In the
Mail Server section, fill in the following:
Hostname: smtp.gmail.com Port: 465
Username: (place your entire gmail address here)
Password: (place your gmail's password here)
Encryption: (pick SSL)
(Interestingly, these options were blank when I went in to do the above. Not sure why they were blank as I know I filled them in a few years ago. So somehow, they got "blanked out" recently.)
Secondly, you WILL have to set your gmail account settings to "Allow access for less secure apps". See this link:
https://support.google.com/accounts/answer/6010255 read if you want (I wanted to which is why I added this link first) and the you will find a link to access your account settings there. Or if you don't care about the first link, you can go directly to the applicable account setting here:
https://www.google.com/settings/security/lesssecureapps (make sure "Turn on" is selected).
That's it! You can now use your gmail again if you changed the above settings in Calibre and the one setting in your gmail account. Fingers and toes crossed for anyone in this situation. It worked for me and it worked for my father who also suddenly became afflicted with this error. And he lives hundreds of miles from me.