Chuck Moulton on "[Plugin: Postman SMTP Mailer/Email Log] authentication failed error"

I'm getting a lot of emails with the following error message:

Authentication failed: Bad username / password

Half of the emails seem to go through, half don't. But the username / password is the same for all emails sent through my SMTP server (SendGrid), so I don't see why half would authenticate and half wouldn't.

I didn't have this problem with other SMTP plugins for WordPress.

Any ideas what's going on and how to fix it?

https://wordpress.org/plugins/postman-smtp/