Allow for a two machine setup so that a user's mailbox can be located on two servers, such that the two mailboxes are synchronised in an IMAP like fashion. Both mail servers can receive mail via alternate MX records and update the user's local mailbox, which in turn gets synchronised with the corresponding mailbox in the alternate server.
 
Likewise a user can connect to either servers to read, delete or send mail and the other server's corresponding mailbox will get synchronised accordingly.
 
DNS caching could be limited to say 10 minutes, so within 10 minutes or less after the DNS gets changed, users would get redirected to the alternative site.  After the problem is fixed, the failed server resynchronises the files and the DNS entries get reversed and we are back to normal.
 
The result is a redundant fully distributed email system and you can charge for two email servers, since they are both active.  OK, if you could guarantee that only duplicated mail boxes exist, you might want to charge 50% for the backup server or make it free since the total active mailboxes will not exceed the licensed amount :)

Customers migrating from other Mail platforms often have a set of per user PST files they wish to migrate over to MDaemon. Currently the method of migrating this information involves each user setting up a new Outlook connector profile, Mounting the PST and moving the data over manually.

This works but is long winded for multiple users. It would be great if this migration could happen at the server. For example. you could have a migration tool that matches PST files (named to match MDaemon user accounts) and runs through the process of importing the data within the PST file storing it under that account. In a similar way that the Exchange Migration tool works.
The ability to migrate for other mail platforms would help attract new customers who wish to try out all of the benefits of MDaemon and Outlook Connector.

I have been testing MDaemon & other email server software in order to put them into production in our office.

MDaemon has two ways of sharing emails : Public folders & shared folders with their own ACLs. In public folders the flags can be either per user or shared flags with other users. If the flags are shared with other users, then the Read, Unread, Replied, Forwarded, etc. all status are shared with other users, this way one can know which emails have been replied or forwarded. However if the Public Folders are configured with Per User IMAP flag settings, then every user does get to know which emails are new and unread by him, but it does not reflect whether a particular email has been replied or forwarded by any of his colleague. What I am looking is for a situation where all flag status are shared between the users except for the read / unread status. This way every user will have a clear picture of the conversation as well as what mails are new to him. This is a feature that works well with other mail servers such as Axigen & Desknow, where all flags are shared except for read unread status which is configured on a per user basis.

Implement a fully functional Corporate signature system:
1. Allows a full set of macros to be used so that a fully dynamic signature can be built rather than having to rely on on a mixture of both Outlook and MDaemon. Things like $FirstName$, $LatsName$, $JobTitle$, $PhoneNumber$, $Department$
2. Allow macros to pull in Active Directory user data.
3. Build in ability to position signature at the end of your reply rather than bunching them all up at the end of a long message thread... it looks very unprofessional!
4. Perhaps in conjuction with the Outlook NewMail/Reply feature, use the CF search and Replace to allow different corporate signatures to be appended (in the correct position).
5. Keep in mind the need for marketing departments to be able to add their blurb to emails too... this is becoming more and more important, so while the functionality is there with CF already, it needs to be tidied up so that positioning is correct, as per corporate signature.