Domain Alias vs Mailbox: The Difference That Decides Your Bill

The difference between a domain alias and a mailbox is what you log into and what you pay for. A mailbox is a real inbox: it has a login, storage, a bill, and a person (or a function) that reads it. A domain alias is routing: mail addressed anywhere on a second domain lands in a mailbox that already exists. Alias addresses do not have their own storage, password, or price. Understand that split and most multi-domain email decisions make themselves.

Disclosure: we make SuperMailOS, a host built around exactly this distinction, so we will use it as an example. The mechanics are universal.

What a mailbox actually is

A mailbox is the account. It holds the messages, the folders, the search index. When you check mail, you authenticate as the mailbox. Storage quotas, retention, and login security all attach here. And on nearly every provider, the bill attaches here too: per-user pricing means per-mailbox pricing, which is why understanding what creates a mailbox matters for cost. In SuperMailOS the rule is stated plainly: a mailbox is a person; you pay per person, not addresses, and the authentication primer covers what each domain's records must carry.

What a domain alias is

A domain alias points a whole second domain at your existing mail. You own acme.app with a real mailbox, and you also own acme.tools. Making acme.tools a domain alias of your account means hello@acme.tools and support@acme.tools deliver into the mailbox you already read, without any new logins. Nothing is stored "in" the alias. Delete the alias and the domain stops delivering; the mailbox is untouched.

The same idea exists one level down: a plain email alias (hello@acme.app delivering into your one mailbox) is the per-address version. We compared that pattern against catch-alls in catch-all vs email aliases; the spam side of that trade-off is covered in catch-all email spam risk.

Domain alias vs mailbox, side by side

• Login: a mailbox has one. An alias has none; it inherits whoever it delivers to.
• Storage: a mailbox owns its quota. An alias adds zero.
• Replying: from a mailbox, you reply as yourself. From an alias-delivered message, you reply as the alias address if your host supports it (in SuperMailOS, reply as any address on any domain).
• Billing: on per-user hosts, each mailbox is a line item. Aliases are usually free or cheap, because they are routing, not accounts.
• Lifecycle: deleting a mailbox deletes mail. Deleting an alias deletes nothing but the route.

The decision, in practice

Use one mailbox per human or function that reads mail, and aliases for every address anyone might send to. A solo founder with three products does not need three mailboxes; they need one mailbox, three domain aliases, and a dozen address aliases funneled into it. That shape is why "how many mailboxes do I need" reduces to "how many people read mail," a question with a small number as the answer. If you are scaling this across products, our guide to one inbox for multiple domains lays out the three common architectures, and email hosting for multiple domains covers the operator view.

Where providers differ

Watch two things when comparing hosts. First, whether domain aliases are unlimited or metered, since the whole pattern collapses if every alias costs a mailbox's fraction. Second, whether replying as an alias address is supported, because an alias you cannot reply from is a one-way door: fine for inbound-only addresses, miserable for support@. These are product decisions, not protocol limits, and they vary wildly; the comparison in flat rate vs per user pricing shows how the billing models interact with them.

Frequently asked questions

Is a domain alias the same as email forwarding?

Close, but forwarding usually means sending mail onward to a different provider's address, while a domain alias routes within your mail setup. The practical difference: forwarding can break the original recipient when you switch providers, and replying as the original address is harder.

Do aliases cost money?

On most hosts, address aliases are free or near-free. Domain aliases vary: some plans include them, some meter them. On SuperMailOS, aliases of both kinds are free and unlimited on every plan; you pay per mailbox only.

Can a mailbox have more than one domain?

Yes, that is the multi-domain pattern: one mailbox receiving for several domains, each contributing aliases. The mailbox stays single; the addresses multiply.

What happens to mail when I delete an alias?

Delivery to that address stops immediately. Mail already delivered into the mailbox stays. If the address was receiving abuse, deleting the alias is the clean off-switch.