A Kerio Connect customer has to accept zip files from his customers. Unfortunately, he also gets a very large number of malware laden zip attachments. What to do?
The simplistic answer is to have the customers rename their attachments. Realistically, that's not possible. Some of the customers are stubborn, some are forgetful, but often the production of these messages is done by software and therefore the customer is understandably unwilling to change their procedures because one destination doesn't like zip files.
Another answer is to not use email. Setup an ftp server or use one of the many public file sharing services. Again, customers may resist this change.
Block, but forward
The solution my customer came up with is to block attachments but forward the original email (with the attachment) to another account. He uses a Gmail account for this, but you can use an address on your own server (at the cost of an extra license, of course).
The original recipient gets the message with the zip file stripped - assuming, of course, that it did not run afoul of any other spam/virus issues. They know that they are missing a legitimate attachment, so they ask their manager to retrieve it. The managers all have the "badzips" password, so they login, find the message and then save the message attachment, and put it on some convenient shared drive. It's easy to find because the original recipient knows the Subject: line and anything else needed to identify the right message.
Originally, both my customer and I expected reluctance and complaints from the users, but that didn't turn out to be the case: apparently, they actually like this method!
My customer does something similar with email that has exceeded the "block" Spam score. He uses a different address, of course and only he has access to that account. It's seldom used - only when some Very Important New Customer not yet white-listed has had their email rejected, but when needed, it's very useful.
This might let you lower your spam/block limits to be more aggressive without losing anything forever.