Hello,
When you enable "AntiSpam/AntiVirus (recommended): ClamAV, Amavis & Spamassassin, Requires 1Gb+ RAM"
The mail send sign DKIM key two times. Below is log
cat /var/log/maillog
Aug 8 23:20:11 server1 postfix/smtpd[22593]: connect from localhost[127.0.0.1]
Aug 8 23:20:11 server1 postfix/smtpd[22593]: 72D88E07B0: client=localhost[127.0.0.1], sasl_method=LOGIN, sasl_username=email@yourdomain.com
Aug 8 23:20:11 server1 postfix/cleanup[22646]: 72D88E07B0: message-id=<ef240cb8f48b4c21dd3a3605d00c966b@yourdomain.com>
Aug 8 23:20:11 server1 opendkim[21907]: 72D88E07B0: DKIM-Signature field added (s=default, d=yourdomain.com)
Aug 8 23:20:11 server1 postfix/qmgr[21897]: 72D88E07B0: from=<email@yourdomain.com>, size=589, nrcpt=1 (queue active)
Aug 8 23:20:11 server1 postfix/smtpd[22593]: disconnect from localhost[127.0.0.1]
Aug 8 23:20:12 server1 postfix/smtpd[22696]: connect from unknown[127.0.0.1]
Aug 8 23:20:12 server1 postfix/smtpd[22696]: 18D6CE07C1: client=unknown[127.0.0.1]
Aug 8 23:20:12 server1 postfix/cleanup[22646]: 18D6CE07C1: message-id=<ef240cb8f48b4c21dd3a3605d00c966b@yourdomain.com>
Aug 8 23:20:12 server1 opendkim[21907]: 18D6CE07C1: DKIM-Signature field added (s=default, d=yourdomain.com)
Aug 8 23:20:12 server1 postfix/qmgr[21897]: 18D6CE07C1: from=<email@yourdomain.com>, size=1408, nrcpt=1 (queue active)
Aug 8 23:20:12 server1 amavis[21744]: (21744-01) Passed CLEAN {RelayedOutbound}, MYNETS LOCAL [127.0.0.1]:57904 <email@yourdomain.com> -> <test@gmail.com>, Message-ID: <ef240cb8f48b4c21dd3a3605d00c966b@yourdomain.com>, mail_id: 2a-USGxJx-nW, Hits: -1.099, size: 913, queued_as: 18D6CE07C1, dkim_sd=default:yourdomain.com, 591 ms
Aug 8 23:20:12 server1 postfix/smtp[22647]: 72D88E07B0: to=<test@gmail.com>, relay=127.0.0.1[127.0.0.1]:10024, delay=0.71, delays=0.09/0/0.05/0.57, dsn=2.0.0, status=sent (250 2.0.0 from MTA(smtp:[127.0.0.1]:10025): 250 2.0.0 Ok: queued as 18D6CE07C1)
Aug 8 23:20:12 server1 postfix/qmgr[21897]: 72D88E07B0: removed
Aug 8 23:20:12 server1 postfix/smtp[22657]: 18D6CE07C1: to=<test@gmail.com>, relay=gmail-smtp-in.l.google.com[173.194.205.26]:25, delay=0.79, delays=0.05/0/0.56/0.18, dsn=2.0.0, status=sent (250 2.0.0 OK 1533784812 i3-v6si672583qkf.13 - gsmtp)
Aug 8 23:20:12 server1 postfix/qmgr[21897]: 18D6CE07C1: removed
Raw header of Gmail shows