No, the proper response to this is to fork out more money for a certificate that is more widely supported! It's email, it should be secure / trustworthy?
This might be an issue with - "purchased certificate must be combined with a root CA certificate for it to be trusted by browsers", see https://devcenter.heroku.com/articles/ssl-certificate i think it is an nginx thing? And definitly do NOT disable ssl
Disabling SSL is even worse - there is no way I'm going to trust someone with something as important as my email if they don't offer SSL or can't get it right.