Hi, please update the addon to version 1.2.2, the fatal error management can be set on the settings panel. Version 1.2.1 treats error as fatal. Version 1.2.2 treats errors in the old way but the behaviour can be changed.
In your case, you have an overquota or by the smTp service or by the receiving domain, and not managing the fatal error means the addon continues to send, but your SMTP service then delivers no emails and they’re simply dropped.
Anyway, if you prefer the addon not to stop the sending process, just check the settings (it should already set to not stop on fatal errors.