There are several potential reasons why you are not receiving the email in question. To know with certainty what the exact reason is we will need to see the bounced messages that the sender is receiving, if any. If the sender gets a bounce-back to their message please forward this to support@alentus.com. This will help us to identify the cause of this problem.

Once reason why email is not being received is that it has been classified as being "SPAM" by our Barracuda Mail Filters/CanIt Spam Filters. Another likely reasons is that our mail server is rejecting the incoming email based on the fact that the sending server is listed in one or more of the RBLs (Realtime BlackLists) that Alentus subscribes to. RBLs are used to identify sources of spam and are used to limit the amount of spam received by our customers.  

If it is determined that this is the problem there are a few potential solutions, as follows:

1. Have the sender send their email via a different SMTP server.
2. Have the sender send their email from a different email address.
3. Have the administrator of the sending SMTP server fix their server such that it is no longer blacklisted.
4. We can disable spam filtering for your domain to ensure all email arrives, but this will lead to a significant increase in the amount of spam you receive.  

The RBL that Alentus currently uses is zen.spamhaus.org (http://www.spamhaus.org/zen/). You can check if your SMTP server is on a given blacklist by using the following link: http://mxtoolbox.com/blacklists.aspx. You will need to know the IP address of the sending server to check the blacklist.