When a HappyFox account is deleted, the associated forwarding email ID of that account is also deleted. 

It is your responsibility to maintain and remove the forwarding rule from all the mailboxes that were added as categories on your HappyFox account. This needs to be done on your mail hosting service.

If the forwarding rule is not removed, and someone sends an email to yourmailbox@yourcompany.com then HappyFox servers will reject that message and send an email like below:

Delivery to the following recipient failed permanently:

accountname-xxxxx@happyfox.com

Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the server for the recipient domain happyfox.com by mail.happyfox.com. [54.213.101.90].

The error that the other server returned was:
550 5.1.1 <accountname-xxxxx@happyfox.com>: Recipient address rejected: Your HappyFox account is no longer available.

This is to indicate that the forwarding email ID associated to your HappyFox account is not valid anymore.