There are different webmails that can be used with Sendmail MTA (or Postfix), but my favourite is SquirrelMail (see older posts).
Now I will try RoundCube (here). The best thing is that both can be used on the same server!
Where are mail folders :
INBOX – /var/spool/mail/username
DRAFTS (in RoundCube it is called INBOX.Drafts folder) – /home/username/mail/INBOX.Drafts
SENT (u RoundCube it is called INBOX.Sent folder) – /home/username/mail/INBOX.Sent
TRASH (u RoundCube it is called INBOX.Trash folder) – /home/username/mail/INBOX.Trash
Where are personalization data (name, surname, mail address, Reply To, signature, address book, color/style) :
SquirrelMail : /var/lib/squirrelmail/prefs/username.pref+abook+sig
RoundCube : personalizations/preferances are in a MySQL DB formed before RoundCube installation
(Take care! ROundCube Identities may cause problems if viewed through Internet Explorer 8, but FireFox 14 has no problems!)