With the inherent REDDOXX certificate authority (CA), certificates for your e-mail aliases can be created automatically by the Appliance on demand.
The advantage is that you save the costs for purchasing certificates as well as for administration.
The disadvantage is, that the mail recipient must have imported your root certificate once to recognize the validity of your certificates.
Another disadvantage is, that so created certificates can not be checked with CRL or OCSP.
To make it easier for your communication partner to exchange your root certificate, you can provide your root certificate on one of your web servers for download.
It would be best if your web server is equipped with an SSL certificate, so that your communication partner can trust the root certificate he wants to download there.