text: Title: dkim missing via auth-smtp (submission), but added via local shell mail only
  Body: # Impacted versions
  
  * OS Type: Debian
  * OS Version: Debian 10 (buster)
  * Database Type: PostgreSQL
  * Database version: 11.19
  * Modoboa: 2.0.5
  * installer used: Yes
  
  # Steps to reproduce
  Standard configuration via Installer.
  /etc/postfix/main.cf contains:
  ```
  smtpd_milters = inet:127.0.0.1:12345
  non_smtpd_milters = inet:127.0.0.1:12345
  milter_default_action = accept
  milter_content_timeout = 30s
  ```
  opendkim.service is configured with `Socket inet:12345@localhost` and is running.
  
  Domain has a dkim key and has a green dkim icon.
  
  # Current behavior
  
  When sending an email with the correct From header from a mail client, no dkim header is added. The mail.log has no dkim header:
  ```
  Mar  6 11:45:45 modoboa1 postfix/submission/smtpd[29468]: connect from gw.apg2.net[81.3.13.187]
  Mar  6 11:45:45 modoboa1 postfix/submission/smtpd[29468]: Anonymous TLS connection established from gw.apg2.net[81.3.13.187]: TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)
  Mar  6 11:45:45 modoboa1 postfix/submission/smtpd[29468]: NOQUEUE: client=gw.apg2.net[81.3.13.187], sasl_method=PLAIN, sasl_username=user@apg2.de
  Mar  6 11:45:47 modoboa1 postfix/smtpd[29508]: connect from localhost[127.0.0.1]
  Mar  6 11:45:47 modoboa1 postfix/smtpd[29508]: 7E631BFBB1: client=localhost[127.0.0.1], orig_client=gw.apg2.net[81.3.13.187]
  Mar  6 11:45:47 modoboa1 postfix/cleanup[29509]: 7E631BFBB1: message-id=<3E50EF74-D3C3-439B-AFC1-58EE779DFB83@apg2.de>
  Mar  6 11:45:47 modoboa1 postfix/smtpd[29508]: disconnect from localhost[127.0.0.1] ehlo=1 xforward=1 mail=1 rcpt=1 data=1 quit=1 commands=6
  Mar  6 11:45:47 modoboa1 postfix/qmgr[25853]: 7E631BFBB1: from=<user@apg2.de>, size=4655, nrcpt=1 (queue active)
  Mar  6 11:45:47 modoboa1 amavis[18307]: (18307-14) Passed CLEAN {RelayedOutbound}, ORIGINATING LOCAL [81.3.13.187]:37543 [81.3.13.187] <user@apg2.de> -> <user@ymail.com>, Message-ID: <3E50EF74-D3C3-439B-AFC1-58EE779DFB83@apg2.de>, mail_id: MhInoSFVu_r4, Hits: -2.899, size: 4179, queued_as: 7E631BFBB1, 1564 ms
  Mar  6 11:45:47 modoboa1 postfix/submission/smtpd[29468]: proxy-accept: END-OF-MESSAGE: 250 2.0.0 from MTA(smtp:[127.0.0.1]:10025): 250 2.0.0 Ok: queued as 7E631BFBB1; from=<user@apg2.de> to=<user@ymail.com> proto=ESMTP helo=<[10.5.248.3]> sasl_username=<user@apg2.de>
  Mar  6 11:45:49 modoboa1 postfix/smtp[29511]: Trusted TLS connection established to mta6.am0.yahoodns.net[67.195.228.110]:25: TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256
  Mar  6 11:45:51 modoboa1 postfix/smtp[29511]: 7E631BFBB1: to=<user@ymail.com>, relay=mta6.am0.yahoodns.net[67.195.228.110]:25, delay=3.6, delays=0.04/0.03/2.1/1.4, dsn=2.0.0, status=sent (250 ok dirdel)
  Mar  6 11:45:51 modoboa1 postfix/qmgr[25853]: 7E631BFBB1: removed
  ```
  
  An email sent locally via a linux shell mail tool gets the dkim header though:
  ```
  Mar  6 12:27:42 modoboa1 postfix/pickup[25852]: 4A49BBFD04: uid=1000 from=<user>
  Mar  6 12:27:42 modoboa1 postfix/cleanup[17156]: 4A49BBFD04: message-id=<20230306112742.GA17268@apg2.de>
  Mar  6 12:27:42 modoboa1 opendkim[1502]: 4A49BBFD04: DKIM-Signature field added (s=modoboa, d=apg2.de)
  Mar  6 12:27:42 modoboa1 postfix/qmgr[25853]: 4A49BBFD04: from=<user@apg2.de>, size=464, nrcpt=1 (queue active)
  Mar  6 12:27:43 modoboa1 postfix/smtp[17163]: Trusted TLS connection established to mta6.am0.yahoodns.net[67.195.204.77]:25: TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256
  Mar  6 12:27:44 modoboa1 postfix/smtp[17163]: 4A49BBFD04: to=<user@ymail.com>, relay=mta6.am0.yahoodns.net[67.195.204.77]:25, delay=2.2, delays=0.06/0.01/1.3/0.82, dsn=2.0.0, status=sent (250 ok dirdel)
  Mar  6 12:27:44 modoboa1 postfix/qmgr[25853]: 4A49BBFD04: removed
  ```
  
  # Expected behavior
  
  Mails sent via authenticated-smtp from an external mail client should get the dkim signature header.
  