Certain Emails not fetched

[iscon]

One of my customers cannot receive emails from a particular sender (it's a fax delivery service). It works fine on TB and Gmail. Headers all look normal, the only slight oddity is that the sending email address as well as the sending server has hyphens and numbers in the name. I tested sender names with 2 hyphens but in my case all seems normal. Unfortunately I cannot test the original sender since email delivery is hardwired to the sender of the fax.
Any idea where to look?