When we ran our own mail server, we would be dealing with spam issues at least 10 hours a week. It could be the president got a new spam and flagged it as ham and now he's getting even more spam, or that someone messed with the spamassasin configuration or that someone was flooding our system with more mail then our small t1 could handle (we have a much much much larger connection now).
We then tried baracuda spam filtering, and it worked a little easier, but still far too many issues. We tried to send a news letter out once (to subscribed prospective students who asked for information) and got flagged and put on a black hole list. The best was when we switched to AT&T and found out the entire IP block they gave us (a class C) was black listed as being spammers. After not being able to send email for 2 weeks we had to get AT&T to give us a new block of IP addresses.
Finally, storage became a problem, as we got bigger and bigger I needed more storage, but they didn't want to pay for more storage, so we started limiting the size of the inbox. This caused more complaints and even more support calls. As the mail grew, we had to restrict them more and more. All this on top of trying to maintain antivirus for inbound and outbound mail with a ever increasing amount of email. We ended up with a cluster of servers that were just costing us more time and more money each year.
The breaking point was when the president asked to have his contacts, mail, and calendar sync across multiple devices. At this point running IMAP, SMTP, and using calendaring built into our web portal just wasn't going to cut it, we needed to move to where the big boys play. First we looked at exchange, but the advanced features the staff wanted would cost us, and we would have to setup a AD environment and bridge it to novell. Next was zimbra, but the cost was almost identical to running exchange. Finally we called google. Being a EDU has it's benefits as they were 100% free.
No spam to deal with, no servers to run, no storage to manage, no black lists to avoid, no virus scanning to deal with. Just email, calendaring, and docs to everyone on almost every device. Best of all we got remote wipe functionality on some devices and our support calls on email dropped to less than 10 a year.