cc -fPIC -shared -Wall -Wextra -DPLUGINNAME=antispam -DHAVE_CONFIG_H - I/opt/local/include/dovecot/ -I/opt/local/include/dovecot/src/ -I/opt/ local/include/dovecot/src/lib/ -I/opt/local/include/dovecot/src/lib- storage/ -I/opt/local/include/dovecot/src/lib-mail/ -I/opt/local/ include/dovecot/src/lib-imap/ -I/opt/local/include/dovecot/src/lib- dict/ -I/opt/local/include/dovecot/src/lib-index/ -I/opt/local/include/ dovecot/src/imap/ signature.o antispam-plugin.o antispam-storage.o
dspam-exec.o -o lib90_antispam_plugin.so Undefined symbols: "_mailbox_get_name", referenced from: _mailbox_patternmatch in antispam-plugin.o _mailbox_is_spam in antispam-plugin.o _mailbox_is_trash in antispam-plugin.o _mailbox_is_unsure in antispam-plugin.o
Looks like your linker is prepending _ to each symbol, which probably don't exist since dovecot itself exports w/o the underscore ... I have no idea why it's doing that though.
johannes