Comments from host's tech support...
email@example.com 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 firstname.lastname@example.org
Even though you authenticate as email@example.com for the SMTP, you try to send as firstname.lastname@example.org 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.