Off topic: Is there any problem with buying a certificate and using the same certificate for your postfix TLS email authentication that you use for your web se

Off topic: Is there any problem with buying a certificate and using the same certificate for your postfix TLS email authentication that you use for your web server authentication? I would like to have authenticated email, to minimize the chance of my emails bouncing, and I was hoping to only purchase one certificate. I can't find anybody talking about mail server TLS email authentication on the certification websites. They only talk about web authentication and a bit about email client authentication.