Good Free mailserver wanted - Not Officemail(problems)

I work for a community adive charity with about 25 email accounts.

We have been using Officemail by Burrotech but very often it locks up
or stops and requires a restart. Not very convenient. A longer
collection time has help but at 20 minutes problems still occur.

The remote administration via a browser is helpful to but this causes
problems with lock ups to.

Can others please reccommend any other free mailserver that could
replace Officemail.

Chris.... S.London