Hi OAEPauli,
I was having the same issue and stumbled on another thread that highlights some solutions.
https://www.ads-software.com/support/topic/gmail-smtp-error-could-not-authenticate
Specifically, this worked for me.
“This is SocksCap64.
I got this error too and now it’s works fine.
record here may help the other people meet this error.
1, open https://myaccount.google.com/
at the tab ‘Signing in’ you will find ‘Access for less secure apps’ is turn off.
change it to ‘turn on’
2, open https://accounts.google.com/DisplayUnlockCaptcha
it’s will response:
Account access enabled
Please try signing in to your Google account again from your new device or application.
the problem is resolved.”
Hope that helps.
