Hello.

The issue in your case is a missing MX record.
Sending emails does not require a MX record, but in order to receive the email servers need to know to which IP to direct the email.
Try contacting the support department at onlydomain.com for further assistance because they do not seem to have a tutor on their website. Or at least I couldn't find one :)

Hope it helps,
Denis Rendler

I could not find a solution too on receiving emails from other emails addresses.
Problem occurs when using the Custom nameserver settings -(Using Godadday.com).

I think DigitalOcean should come up with a way or a solution to receive emails by adjusting the MX-Records setting .