The sixty-second answer
Yes, and one account is usually the right answer. Share the login, the storage and the software. Keep each business its own domain, its own authentication records, its own signature and its own customer list. One login is fine; one identity is not, and the identity is the part regulators, receivers and customers actually see.
Two businesses is normal, not unusual
Canada's business population is overwhelmingly small - as of December 2024 there were 1.10 million employer businesses, and 1.08 million of them, 98.2 per cent, were small [1]. In a country of that shape, one person running a trade during the week and a side venture on weekends is not an edge case. It is Tuesday.
So the question is not whether you are allowed to. It is which parts of the setup can be shared without causing trouble later. There is a clean line, and it runs between the plumbing and the identity.
What you can share safely
The account and the bill. Nothing about mail hosting requires a separate subscription per company. One account holding two domains is simpler to administer and simpler to pay for.
The mailbox and the login. If the same human answers both, one mailbox with two identities configured in it is far better than two mailboxes you have to remember to check. The failure mode of two mailboxes is not confusion, it is neglect - one of them goes unread for a week.
The device setup. One phone, one laptop, one set of credentials. The general guidance on keeping devices consistent is in business email on your phone and laptop.
Storage. Two businesses' mail in one mailbox is one storage pool, which is usually cheaper than two allocations sized for peak. The sizing question is worked through in how much email storage a business needs.
What must stay separate: the domain
Each business needs its own domain name, and this is the decision everything else hangs off.
A domain is not cosmetic. It is the unit that mail delivery is addressed to - a sending server must find a usable MX record for the name or treat the situation as an error [2]. It is the unit that authentication attaches to. And it is the unit your customer reads as "who is this from".
The tempting shortcut is to run the second business as an alias on the first domain: [email protected]. Resist it. An alias expansion "simply replaces the pseudo-mailbox address in the envelope with each of the expanded addresses in turn; the rest of the envelope and the message body are left unchanged" [2] - the message is still, in every technical and visible sense, from the first domain. Aliases are the right tool for sales@ versus support@ inside one business, as set out in addresses, aliases and shared mailboxes. They are the wrong tool for two businesses with different customers.
Register each domain in the name of the business that trades under it. If one is incorporated and one is a sole proprietorship, that distinction should be visible in the registrations. Both should have their own registrar contact address that does not live on the domain itself - the reason why is in what happens to your email when your domain expires.
What must stay separate: authentication
Each domain needs its own SPF, DKIM and DMARC records. These are not transferable, and there is no version of "I set it up on the main domain" that covers the second.
DKIM makes the reason clear: it "permits a person, role, or organization that owns the signing domain to claim some responsibility for a message by associating the domain with the message", and it deliberately "separates the question of the identity of the Signer of the message from the purported author" [3]. Signing is a per-domain act. DMARC then requires the visible From domain to be aligned with an authenticated identifier [4], so a message that says it is from business B while being signed only by business A's domain is exactly the mismatch DMARC exists to catch.
The practical result of skipping this is that the newer business's mail goes to junk while the older one is fine, and the owner concludes the new venture is cursed. It is not; it is unauthenticated. The setup walkthrough is SPF, DKIM and DMARC explained for small business, and the symptom is covered in why your business email goes to spam.
If you do use a subdomain rather than a second domain - reasonable for a product line, questionable for a separate company - know that DMARC handles inheritance explicitly. The "sp" tag sets the requested policy for subdomains, and where it is absent the policy in the "p" tag must be applied to subdomains as well [4]. Convenient, and also a statement that the two are the same organisation.
What must stay separate: the names themselves
Choose two names that a customer could not confuse, and check the second against the first before you commit to it.
The Trademarks Act treats confusion functionally rather than by category. Use of a trademark causes confusion with a trade name where use of both in the same area "would be likely to lead to the inference that the goods or services associated with the trademark and those associated with the business carried on under the trade name are manufactured, sold, leased, hired or performed by the same person" - and that applies "whether or not the goods or services are of the same general class" [5]. Two ventures with adjacent names in one town is the scenario the section describes, and being the same owner does not make it tidier when a third party's mark is in the picture.
What must stay separate: the records trail
Two obligations sit underneath the mail.
Books. Every person carrying on business must keep records and books of account in a form that enables tax payable to be determined, retained until six years from the end of the last taxation year to which they relate; where those records are kept electronically, they must be retained in an electronically readable format for the same period [6]. Email is where a great deal of that lives - quotes, purchase confirmations, supplier invoices. Filing by business at the time it arrives costs nothing; separating a merged inbox afterwards costs a weekend. The wider treatment is business email retention and records in Canada.
Customer information. This is the one people get wrong. PIPEDA's Schedule 1 requires knowledge and consent for collection, use and disclosure, and provides that personal information shall not be used or disclosed for purposes other than those for which it was collected except with the individual's consent [7]. An individual is also entitled, on request, to be informed of the existence, use and disclosure of their information [7]. A customer who gave their address to business A has not agreed to hear from business B. One combined mailing list is a disclosure decision disguised as a convenience - see business email privacy obligations in Canada.
The practical shape
One account. Two domains. One mailbox with two send-as identities, each with its own signature and its own reply-to. Server-side rules that file incoming mail by which domain it arrived at, so the sorting is done before you read it. Two folders, two signatures, two customer lists, one login.
Add a second mailbox only when a second person joins one of the businesses, or when one venture generates enough mail that mixing it with the other is genuinely confusing. The counting rule is in how many email addresses your small business actually needs.
Where we sit
MapleMail hosts multiple domains under one account. Each domain gets its own MX, SPF, DKIM and DMARC records, and we hand those to you in writing so neither business is dependent on us to prove who it is. A single mailbox can send under either identity, so you are not buying a seat to gain a signature.
What we will not do is pretend the separation is only technical. The domain, the authentication and the customer list are the three things that make two businesses two businesses; the login is not one of them. Plans and what each mailbox includes are on the pricing page.