Open source options tend to require a bunch of knowledge to set up but after that you generally are equipped with that knowledge to solve any configration issue. I wrote "configuration issue" because aside from "ran out of disk space" none of them really "broke", so it was never "fixing the thing", just the usual mail server admin stuff of figuring out why this or that server didn't like us.
A lot of the bad situations I helped bail other contractors out of, when it came to Exchange, related to disk full conditions, a lack of understanding how transaction logging in the database worked, and database inconsistencies brought on by hardware RAID controller issues or hard power-offs. Very nearly all of my ugly ESEUTIL and ISINTEG experience came from people not respecting the product's storage needs.
Sysadmins who were used to running file servers and thought Exchange was the same thing were the problem. A DBA was actually more apt to understand the product's storage needs because it wasn't wholly dissimilar from the RDBMS they supported.