Email messages inconsistently signed

[lnicola]

• a Welcome to the "postgis-tickets" mailing list message I got was not DKIM-signed
• a [SCM] PostGIS branch master updated. 3.4.0rc1-749-g42f04a29e message was signed, but with a broken signature:

    dkim=fail (message has been altered, 2048-bit rsa key sha256)
      header.d=osgeo.org header.i=@osgeo.org header.b=HFXPb4eW
      header.a=rsa-sha256 header.s=mail;
    dkim=pass (2048-bit rsa key sha256) header.d=osgeo.org
      header.i=@osgeo.org header.b=B8QP3oc/ header.a=rsa-sha256
      header.s=mail;

• [SAC] [OSGeo] #3013: Submission service to send formal emails (was: Mail Submission Agent service to send formal emails) was not signed at all