You receive an error message saying:

Error expanding recipient "<email address>"
Nesting Level too deep (51)
Check the configuration for loops

(Note the '51' number may be slightly different - that's the nesting level - anything over 50 is detected as an error)

This error means that VPOP3 has detected a routing loop in its configuration.

For instance, if you have a user 'A' who is forwarding to user 'B', and user 'B' is forwarding to user 'A', you have a loop. VPOP3 will try to follow the configuration for a while before realising it is never ending (once it reaches over 50 steps down the configuration) so it will report this error message.

Note that you can have more complex loops, such as user 'A' forwarding to list 'B' which forwards to user 'C' who forwards to user 'A'.

To solve the problem, you need to start at the reported email address, and follow the VPOP3 configuration through for that email address/user, and any forwards which apply to that user, and then do the same with the forwarded target user(s), and so on, until you discover the loop.