How can you tell the mail is trusted and the attachment is not infected if you haven’t seen them?
What os are you using?
What version of avast are you using?
What vps version?
What is avast telling you if you try to look at that email?
What is telling you the customers system is not infected?
What email client are you using?
etc etc etc.

Keep in mind that we can not see what is happening on your system, so you have to provide information.