[21.0.4] Unable to send mails encrypt using S/MIME certificates

[tuxmaster5000]

Because reopen will not work, here the link to the original report.
#1090
Here the current log:

util gpgme:MESSAGE:2021-06-01 05h34.22 utc:1632912: Setting GnuPG dir to '/var/lib/greenbone/gvmd/gnupg'
util gpgme:MESSAGE:2021-06-01 05h34.22 utc:1632912: Using OpenPGP engine version '2.2.20'
event alert:MESSAGE:2021-06-01 07h35.17 CEST:1633182: The alert ePost_foo was triggered (Event: Task status changed to 'Done', Condition: Always)
util gpgme:MESSAGE:2021-06-01 07h35.19 CEST:1633182: find_email_encryption_key: Found matching UID for foo@foo.foo
event alert:MESSAGE:2021-06-01 07h38.08 CEST:1634880: Alert ePost_foo (505b79f8-ba0f-4a39-a63b-a7441b925650) has been modified by gott
event alert:MESSAGE:2021-06-01 07h38.16 CEST:1634890: The alert ePost_foo was triggered (Event: Task status changed to 'Done', Condition: Always)
util gpgme:MESSAGE:2021-06-01 07h38.17 CEST:1634890: find_email_encryption_key: Found matching UID for oo@foo.foo
md manage:WARNING:2021-06-01 07h38.17 CEST:1634890: email_encrypt_smime: encryption fail

Using gpg keys instant of certificates will work.