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?
I don’t know where is the problem but we send SMIME signed mails since years to Avast (support, account manager, marketing, product manager…) without any problem. 
I got that info from at least two employees from Avast support working on diffrent cases.
Mail address is: support@business.avast.com
I guess you will use no different address related to support issues?
I can confirm that we use the same email for support issues.
Update: SMBS-1235 was created to investigate this issue…
Update: It seems that the main problem is related to the class of the used certs. Where our support team will use a class 3 cert I use a class 1 cert.
Doesn’t seem logical for me though but this is the main diffrence…