Recent versions of net-mail/cyrus-imapd beginning from 2.3.14-r3 I guess deny an installation of net-mail/dovecot and other mail servers in parallel. There're many good reasons to allow a side-by-side installation of both cyrus-imapd and dovecot i.e. The ebuild suggests it already: "# all blockers really needed?" Reproducible: Always
Fixed in cyrus-imapd-2.4.8 without a revbump.