How to set up a custom email with Heroku
You need a DNS service like dnsmadeeasy or others mentioned on heroku. From there you need to distribute the different records, e.g. ANAME and CNAME to heroku, CNAME to external asset hosts like cloudfront, MX (and A) to email host.
Am myself in the process of dealing with the "email problem":
(easier) The registrar (domain name provider) offers email hosting, but does not offer ANAME or ALIAS records (common): You can point the nameservers to the DNS service and then point the email back to your registrar. Found registrars quite helpful in general with helping if you need config details.
(more annoying) The registrar does not offer email hosting in the first place or has an all or nothing policy re nameservers (everything with them, or nothing with them, like e.g. easily). In this case you have to find an independent email host. Any recommendations? The setup is analogue.
(perfect) Your registrar offers ANAME or ALIAS records and email hosting. Lucky you!