Can I change the format of outgoing email messages for vendor?

[LBCoEnterprises]

A workflow on a schedule in the gallery sends an email with an xlsx attachment to the vendor.

The email with with attachment never gets to the vendor. 

We have tried several tests and the email will go out to external email boxes just fine, but not to the vendor's email box.

We got a communication from the vendor IT:

"Currently, Mimecast only supports chunking (also known as BDAT), for our journal smarthosts, thus BDAT is NOT supported for inbound and outbound email processing.

The emails must use normal data transfer mode for Mimecast to accept these messages."

Is there anything I can do about this on my end in Alteryx to use 'normal data transfer mode' ?

Does it have anything to do with encryption? Currently I am not using any.