i am looking for an email provider that can service a "whole domain" by delivering all email to any user name at my domain to a single mailbox that can then be accessed in a usual manner (like via secure IMAPS ... IMAP over TLS/SSL).
protonmail.ch can do this. For 4€ per month you can use your own domain, but you need access to the settings of the nameserver which is used for your domain. IMAP/SMTP is supported.
from their description, it seems they only do whole domains the common way, where there is a separate mailbox for each user @ the domain. they can do forwarding, but only for specified names. but i do like their encrypted at rest mailboxes. so i do have "skaperen" at their .com domain (for free). too bad it already gets spam. but i think my nephew leaked it on twitter.