I've run my own mail server since I got my first permanent connection in 1995. It's not hard to keep it up and reliable, especially on a small scale. It's not much harder to keep a large one up (I designed and operated a huge qmail hub in the late 90's).
Everything is on hard drives just as it is at google and all the other providers. I don't use rack and data center distribution for redundancy or even RAID to be honest.
However I do have a DLT and a fire safe (which acts as an anti-theft device as it weighs 200Kg) :)
What I'm saying is there are varying grades of redundancy, backup and reliability. It might be good enough, or might give more confidence than the disclaimers Google offer.
As for functionality, Maildir with your MDA of choice gives you all kinds of rube-goldberg opportunities with text files. No silly "max connections" junk when your phone and your laptop hit IMAP at the same time.
The one thing I really liked when going from procmail/postfix/dovecot to Google Apps was spam filtering. I hated keeping up to date with the various new methods spammers discovered to offer me good times with farm animals.
Today, I get a lot of false positives, especially with the "this is spam because other people marked messages like this as spam" emails from my bank. I miss emails from family members, travel vendors, and even online shipment confirmation messages. I've had to create rules to catch them and move them to folders/labels as well as have a client-side process occasionally search for a whitelist and "rescue" the messages.
I never get spam. I rarely give out my address and if i do, its usually an alias unless i really trust the person. Spam? Delete the alias and get the fucker blacklisted as it deterministically points to the culprit.
I use usenet via eternal-September.org eith an alias and some junk in the address. Has worked fine for years now.
