I regularly exchange emails with someone and their emails always come in with the following warning below the message header:

If this person has sent me an attachment, which is common, I have to press "show content" to be able to get the attachment.

The sender is trustworthy and reliable and I have no way to turn this message off that I can discern.

Does anyone know how to eliminate this message for good?  I have added this sender to my contacts (before you suggest that).

Much appreciated