I'm thinking this isn't possible. I have a couple of domains hosted at winhost and use the winhost provided email hosting for those. I have another domain that is hosted elsewhere (and one that I cannot move to winhost) - can I host the email at winhost even though the domain itself is hosted elsewhere? Thanks for the info.
Somewhat depends on intent. If you only needed email aliases, you can add the domain as a pointer to an existing site (Winhost Control Panel > Sites > the domain name > Domain Pointer), enable "Email Alias" for the pointer domain, then only update the MX and TXT records through the DNS provider. Edit - After having a think and performing a quick test, SPF, DKIM and DMARC can be set up for an email domain alias. The caveat with the email domain alias being that it's obviously sharing the same space and email addresses as the site domain. Though this would be much easier explained in a ticket with specific domains rather than written in general terms. If you're expressly looking for mail hosting with separate email accounts, that does essentially require a separate site account. ie, Ordering a separate site, then updating the MX and adding up to three TXT records (for SPF, DKIM and DMARC) through the DNS provider.