sending messages over NC mail with S/MIME-cert makes signature invalid

[Hubhubhurra]

Steps to reproduce

1. Add a S/MIME-certificate to your mail-account
2. Add a signature to your mail-account
3. Send a message with signature and digital sign with some text in the body
4. See in a e-mail-programm (like thunderbid) in sent messages of your account and often you will see "Digital signature is not valid". This can you see in destination inbox too.

Expected behavior

Sending signed messages will have an valid signature.

Actual behavior

If you "Edit as new massage" and send it again from thunderbird all is O.K. but from NC the error seen again.

If you send a message with only simple ASCII-signs S/MIME will be valid. Sometimes with more text too.

I can't find out what is the source of the problem. I was first exposed source is in the needed conversion from 8bit to 7bit, but I found mails with that and valid S/MIME. Second I had in mind including links form email-addresses make this error, but I'm also found mails with that and valid S/MIME

I don't know if it make a difference when the mail-server is on the same machine where NC is running. On my tests NC-machine has no own mail-server.

Mail app version

4.1.2

Nextcloud version

30.0.5

Mailserver or service

exchange, postfix

Operating system

debian stable

PHP engine version

PHP 8.2

Nextcloud memory caching

'memcache.local' => '\OC\Memcache\APCu', 'memcache.locking' => '\OC\Memcache\Redis', 'memcache.distributed' => '\OC\Memcache\Redis'

Web server

Apache (supported)

Database

MariaDB

Additional info

By sending problem above I ask for a solution problem below:

On which place can I add the root-certificate for validation.