This error message is coming from the Outlook account server where it seems that you have performed to many 'actions/activities' in a given period of time so it triggered thresholding.
The solution to that could be reducing the number of simultaneous connections to your email account to 2, the following guide explains how to do it: https://support.getmailbird.com/hc/en-us/articles/220107707-Long-delay-in-checking-incoming-email
At the moment this is the only solution that you can perform in Mailbird, another way could be contacting Microsoft support.