I had to change our SMTP server today as part of a migration to a cloud service. When I entered the new SMTP information I received the same error you have described. However, I was able to forward a report to myself without an issue. I ran the email check tool as well, and it worked without an error.
While it seems to be working, I would love to have more information to determine the source of the error since there is so little debug information available to track it down. I have opened a ticket with Lansweeper, so we'll see if they can provide more information.