I suggest that something like this is used to specify the conflict with alterntive pkgs which provide IMAP and POP3 daemons (like uw-imap):

###########################################
# Relations with other POP3/IMAP server pkgs (like uw-imap)

# Provide the abstract service names (which are virtual pkg names),
# specify their origin (our pkg name as the epoch + version-release):
Provides: IMAPD = %name:%version-%release
Provides: POP3D = %name:%version-%release

# Conflict with all other real pkgs which provide the same services
# (they should specify the origin the same way, so the epoch-version-release
# of the virtual pkgs POP3D & IMAPD will always differ from that of ours if 
# they are provided by a different real pkg):
Conflicts: IMAPD < %name:%version-%release
Conflicts: IMAPD > %name:%version-%release
Conflicts: POP3D < %name:%version-%release
Conflicts: POP3D > %name:%version-%release

# End of the statements to describe relations with other POP3/IMAP server pkgs
########################################

This is more general (works if there are several such pkgs, not only a pair). This approach is already used in uw-imap-2001a-alt9 (and emacs-gnus-21.2-alt12).
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courier-imap-1.6.2-alt2