2015 UPDATE: I discovered that dovecot now uses MUCH longer passwords than it used to, and the database tables I’d found online WILL FAIL to authenticate (they truncate your passwords!). Fixed below
95% of linux configuration on Debian servers is simple, well-documented, well-designed, easy to do, with only a tiny bit of reading of docs.
Sadly, “making email work” is most of the 5% that’s: nearly impossible, very badly designed, badly packaged/documented. This OUGHT to take an hour or two, in practice it takes ONE WEEK to setup. WTF? In 2014? Unacceptable!
So I took several incomplete/broken guides, dozens of pages of help and advice, and synthesized this complete, step-by-step guide. This should get you the webmail you actually want (!) in an hour or less.