It took a long time but we now use SMIME certs for mailing. Per default we sign our outgoing mails, not encrypt them.
It seems that Avast mailserver are unable to process SMIME signed mails properly since I always get feedback from support that they can't read my mails...
If Avast would not be a security company this would be probably funny but to be honest: What's going on with Avasts mail servers?