Hi, 

I'm running into the same problem - QB 2019 Pro Desktop does not accept my webmail password. 

Desktop 2019 - latest update

Email provider: Siteground

Anti-virus - Kaspersky - I have tried aswell with Kaspersky disabled

Password is correct

Mailserver is inline with siteground instructions:

https://www.siteground.com/tutorials/getting-started/setup-email-on-a-computer/

Getting very frustrated...

Thanks for your help

Rob