Request signature issue

We have an issue with request signature with one particular email. 

This has only recently started happening but individual emails comes back as undeliverable to me and then a separate one to say sent -although it appears on my document list as sent (and she can see them if she logs in ) but does not receive an email to say they are pending for signature.

We have contacted our IT support and the emails are not getting stuck in the anti-virus/malware so they cannot help but insist the email is white-listed. We have no issues with requesting signatures from another other email.

Thank you