I use Dovecot as my mail server, with maildir format mailboxes. It is very easy to make the Sent folder be the same as the Inbox: just symlink the Sent messages folder’s cur subdirectory to be the same as the Maildir’s top-level new directory. This ensures any email placed in the Sent messages folder is magically a whisked off to the inbox.

starvald ~>ll Maildir/.Sent\ Messages/
total 85
drwxr-xr-x 5 majid engineers 11 Apr 20 13:01 ./
drwxr-xr-x 268 majid engineers 278 Apr 20 13:26 ../
drwxr-xr-x 2 majid engineers 2 Oct 14 2007 courierimapkeywords/
lrwxrwxrwx 1 majid engineers 6 Oct 30 2010 cur -> ../new/
-rw-r--r-- 1 majid engineers 33 Jun 15 2011 dovecot-keywords
-rw-r--r-- 1 majid engineers 54 Apr 20 13:01 dovecot-uidlist
-rw-r--r-- 1 majid engineers 400 Apr 20 08:38 dovecot.index
-rw-r--r-- 1 majid engineers 24K Apr 20 08:39 dovecot.index.cache
-rw-r--r-- 1 majid engineers 26K Apr 20 13:01 dovecot.index.log
drwxr-xr-x 2 majid engineers 2 Apr 19 09:43 new/
drwxr-xr-x 2 majid engineers 2 Apr 20 08:38 tmp/