Hi leunam013,
Based on your description, I would like to confirm the following information:
Would you please give me more information about the effect from Bounce Address Tag Validation on sending emails through the faxes?
You can configure SMTP reply in Office 365 for the faxes. Please refer to the following article:
How to configure an SMTP relay for Office 365: http://support.microsoft.com/kb/2600912
Thanks,
Anna Shi
