To maintain an existing email account elsewhere, you need to setup your domain so that DevHub hosts just the website portion of your domain. This is done via an A-Record. Instructions for how to host your site using A-Records can be found on Step #1 here: http://support.devhub.com/hc/en-us/articles/202766916-How-do-I-point-my-Domain-to-DevHub-using-A-Records
Once your site is hosted via A-Records, you will then need to ensure that the MX Records (Mail records) are set on your domain. If you have an email account through your Domain Registrar (i.e. GoDaddy), then most likely these settings are already in place and your email should work.
If not, you will need to add to your domain the MX Records (and possibly an A-Record) provided by your email provider.
newdomain.com. MX 10 mail.newdomain.com