Allow for invoices and requests to come from custom domains [MS-629]

After talking with @tdphillips a long time ago, I'm fairly certain this is all that is required for custom domains for emails.

All tests pass, although mysearches took a few tries.
Shouldn't this just  be `assertEqual`? I think it's working now because strings can be treated as a sequence, but if we are meaning to compare strings, I think it would be better to do so explicitly.