Comments from host's tech support...
firstname.lastname@example.org is the log in account used to authenticate in front of the SMTP server. It is common that the email used for authentication and the "MAIL FROM" are the same. In the current situation there is a definition somewhere in the plugin settings, where the "MAIL FROM" is defined as email@example.com
Even though you authenticate as firstname.lastname@example.org for the SMTP, you try to send as email@example.com which is the reason for this error.
Please check the settings in the plugin or contact the plugin developer to ask them where is the "MAIL FROM" defined.