Problema resolvido - arquivo de soquete incorreto usado na configuração do amavis, fazendo com que as mensagens sejam alteradas.
Obrigado pelo seu tempo.
Eu instalei o postfix + dovecot + mysql + amavisd + spamassasin. Tudo funciona bem (o e-mail é enviado e recebido), exceto que o e-mail recebido fica muito tempo na fila (por cerca de 15 minutos) antes de chegar à caixa de correio do usuário. Se eu correr
# postsuper -r ALL && postfix flush postsuper: Requeued: 1 message
a mensagem é imediatamente removida da fila e enviada com sucesso para a caixa de correio do usuário.
O conteúdo do arquivo /etc/postfix/master.cf é
smtp inet n - n - - smtpd
-o content_filter=smtp-amavis:127.0.0.1:10024
-o receive_override_options=no_address_mappings
pickup fifo n - n 60 1 pickup
-o content_filter=
-o receive_override_options=no_header_body_checks
cleanup unix n - n - 0 cleanup
qmgr fifo n - n 300 1 qmgr
tlsmgr unix - - n 1000? 1 tlsmgr
rewrite unix - - n - - trivial-rewrite
bounce unix - - n - 0 bounce
defer unix - - n - 0 bounce
trace unix - - n - 0 bounce
verify unix - - n - 1 verify
flush unix n - n 1000? 0 flush
proxymap unix - - n - - proxymap
smtp unix - - n - - smtp
relay unix - - n - - smtp
-o fallback_relay=
showq unix n - n - - showq
error unix - - n - - error
discard unix - - n - - discard
local unix - n n - - local
virtual unix - n n - - virtual
lmtp unix - - n - - lmtp
anvil unix - - n - 1 anvil
scache unix - - n - 1 scache
maildrop unix - n n - - pipe
flags=DRhu user=vmail argv=/usr/local/bin/maildrop -d ${recipient}
uucp unix - n n - - pipe
flags=Fqhu user=uucp argv=uux -r -n -z -a$sender - $nexthop!rmail ($recipient)
ifmail unix - n n - - pipe
flags=F user=ftn argv=/usr/lib/ifmail/ifmail -r $nexthop ($recipient)
bsmtp unix - n n - - pipe
flags=Fq. user=foo argv=/usr/local/sbin/bsmtp -f $sender $nexthop $recipient
smtp-amavis unix - - y - 2 smtp
-o smtp_data_done_timeout=1200
-o disable_dns_lookups=yes
-o smtp_send_xforward_command=yes
127.0.0.1:10025 inet n - y - - smtpd
-o content_filter=
-o smtpd_helo_restrictions=
-o smtpd_sender_restrictions=
-o smtpd_recipient_restrictions=permit_mynetworks,reject
-o mynetworks=127.0.0.0/8
-o smtpd_error_sleep_time=0
-o smtpd_soft_error_limit=1001
-o smtpd_hard_error_limit=1000
-o receive_override_options=no_header_body_checks
-o smtpd_bind_address=127.0.0.1
-o smtpd_helo_required=no
-o smtpd_client_restrictions=
-o smtpd_restriction_classes=
-o disable_vrfy_command=no
-o strict_rfc821_envelopes=yes
dovecot unix - n n - - pipe
flags=DRhu user=vmail:mail argv=/usr/libexec/dovecot/deliver -d ${recipient}
vacation unix - n n - - pipe
flags=Rq user=vacation argv=/var/spool/vacation/vacation.pl -f ${sender} -- ${recipient}
e a saída de "postconf -n" é
alias_database = hash:/etc/aliases alias_maps = hash:/etc/aliases broken_sasl_auth_clients = yes command_directory = /usr/sbin config_directory = /etc/postfix daemon_directory = /usr/libexec/postfix debug_peer_level = 2 delay_warning_time = 4 disable_vrfy_command = yes html_directory = no inet_interfaces = all mail_owner = postfix mailbox_size_limit = 2048000000 mailq_path = /usr/bin/mailq.postfix manpage_directory = /usr/share/man message_size_limit = 204800000 mydestination = $myhostname, localhost.$mydomain, localhost mydomain = ubiqom.eu myhostname = mail.ubiqom.eu mynetworks = $config_directory/mynetworks newaliases_path = /usr/bin/newaliases.postfix queue_directory = /var/spool/postfix recipient_delimiter = + relay_domains = proxy:mysql:/etc/postfix/mysql-relay_domains_maps.cf sendmail_path = /usr/sbin/sendmail.postfix setgid_group = postdrop smtp_tls_session_cache_database = btree:$data_directory/smtp_tls_session_cache smtp_use_tls = yes smtpd_client_restrictions = smtpd_data_restrictions = reject_unauth_pipelining smtpd_helo_required = yes smtpd_helo_restrictions = smtpd_recipient_restrictions = permit_sasl_authenticated, permit_mynetworks, reject_unauth_destination, reject_non_fqdn_sender, reject_non_fqdn_recipient, reject_unknown_recipient_domain, reject_rbl_client zen.spamhaus.org, reject_rbl_client bl.spamcop.net, reject_rbl_client dnsbl.sorbs.net smtpd_sasl_auth_enable = yes smtpd_sasl_local_domain = $myhostname smtpd_sasl_path = private/auth smtpd_sasl_security_options = noanonymous smtpd_sasl_type = dovecot smtpd_sender_restrictions = smtpd_tls_cert_file = /etc/pki/tls/certs/mail.ubiqom.eu.crt smtpd_tls_key_file = /etc/pki/tls/private/mail.ubiqom.eu.key smtpd_tls_loglevel = 1 smtpd_tls_received_header = yes smtpd_tls_security_level = may smtpd_tls_session_cache_timeout = 3600s smtpd_use_tls = yes tls_random_source = dev:/dev/urandom transport_maps = hash:/etc/postfix/transport unknown_local_recipient_reject_code = 550 virtual_alias_maps = proxy:mysql:/etc/postfix/mysql-virtual_alias_maps.cf, regexp:/etc/postfix/virtual_regexp virtual_gid_maps = static:12 virtual_mailbox_base = /home/vmail virtual_mailbox_domains = proxy:mysql:/etc/postfix/mysql-virtual_domains_maps.cf virtual_mailbox_maps = proxy:mysql:/etc/postfix/mysql-virtual_mailbox_maps.cf virtual_minimum_uid = 101 virtual_transport = dovecot virtual_uid_maps = static:101
Obrigado antecipadamente!