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OpenDKIM is a DKIM sign and verify software.

OpenDMARC is a DMARC record verify software.

tinc is a VPN (Virtual Private Network) software.

Postfix is a mail transfer agent (MTA).

Dovecot is a mail delivery agent (MDA).

freshclam is a virus database update tool for ClamAV.

ClamAV is an open source antivirus.

Amavis is a content filter for mail.