There can be a few reasons why your contacts may be experiencing difficulties unsubscribing.
It could be because the user has been sent an email to an old email address they used to use, and the IT team has set things up so that emails for that address would be redirected to their current one.
For example, let say my current email address is:
However, all of my old email addresses still work, which are:
The other reason this can happen is because the email was sent to an alias. For example, I get emails redirected to my account which has been sent to:
… and a whole of a lot more!