[Dovecot] Problems compiling PHP --with-imap - can't find rfc822.h

I'm trying to add webmail (IMP/HORDE) to my OpenBSD 3.5 mail server (Procmail/Amavid/SA/DCC/Razor/Dovecot 0.99.10.7), which means adding PHP and several other libraries.

I'm building PHP 4.3 from the ports tree and, as per the IMP/HORDE readme files, added --with-imap to the makefile. Makeing PHP barfs with a "Cannot find rfc822.h. Please check your IMAP installation." error. I did some digging around and this seems to be part of Courier-IMAP and Mutt, but not part of Dovecot. Can this header file just be grabbed from Courier or Mutt and dropped into /usr/include? Is there something else that's got to be done here?