Postfix offers a lot of restriction possibilities to allow or block connections.
smtpd_relay_restrictions - Access restrictions for mail relay control (who can relay?).
smtpd_helo_restrictions - Optional restrictions that the Postfix SMTP server applies in the context of a client HELO command.
smtpd_restriction_classes - User-defined aliases for groups of access restrictions. One major application is for implementing per-recipient UCE control.
smtpd_recipient_restrictions - Optional restrictions that the Postfix SMTP server applies in the context of a client RCPT TO command, after smtpd_relay_restrictions.