Fastmail alias vs. Masked Email

Which is which

An alias is an address you invent and type in yourself, like shop@you.example. It suits a handful of addresses you mean to keep and want to remember.

A Masked Email address is one Fastmail generates for you, like gentle.keys1882@fastmail.com. It suits a fresh address for every signup, because you never have to think one up and you can switch any of them off on its own. Replies go out from it by default, and you can start a message from one too.

Side by side

	Alias	Plus addressing	Masked Email
Who picks the name	You do	You do	Fastmail generates it
Setup before you can use one	Create it in settings	None	Generate it in settings or an app
Hides your real address	Yes	No, the base is in the string	Yes
How many you get	600, plus 15 per user	No limit	Fastmail publishes no number
An off switch that does not bounce	No, only a rule	No, only a rule	Yes, block it
Reversible	Disabled yes, deleted only if the name is free	Not applicable	Yes, blocked or deleted

What the sender finds out

Disabling an alias notifies the sender with an undeliverable bounce, and so does deleting one: Fastmail states that any further email sent to that address will bounce. Either way, whoever wrote to you is told the address is dead, including the senders you made the address to get away from.

Deleting is different if the alias sits at a domain of your own with a catch-all on it. Nothing bounces then, because the address simply stops being one you defined and the catch-all takes it, so the mail keeps arriving. Fastmail's way to stop a single address there is the opposite of deleting: keep the alias and turn on reject all mail sent to this address, which bounces that one and leaves the rest of the domain working.

You can also silence an alias without bouncing, by leaving it switched on and sending its mail to Trash with a rule on the To/Cc/Bcc condition, or by pointing the alias's own delivery options at the same place. Nothing bounces, because Fastmail accepted the message before the rule ran. What you do not get is an off switch: the address still works, still takes whatever arrives, and still counts against your 600.

A masked address has that switch. Blocking it takes the mail to Trash, the sender sees a normal delivery, and switching it back on resumes delivery. Deleting a masked address does bounce, so blocking is the quiet option.

A deleted alias frees the name up, and Fastmail says another user could potentially create the alias in the future if it is at a Fastmail domain. You can make the same alias again yourself, on your own domain or on a Fastmail one nobody has taken in the meantime, but that is recreating it rather than undoing the deletion. A deleted masked address is kept under Review deleted masked addresses in your settings and restores as itself.

How many you get

An account holds 600 aliases, plus 15 for every user in the account. That is a lot if you are picking them by hand, and not many if you make one per signup.

Masked Email has a ceiling too, and a separate limit on how fast you can create new ones, but Fastmail publishes neither number and says only that both exist to prevent abuse. In practice the one-per-signup habit is the one Masked Email is built for and aliases are not.

Where each one lives

Aliases are under Settings, then My email addresses. Choose Add address, then Create an alias, then pick the part before the @ and a domain from the dropdown. Once it exists you can give it a name and a signature for sending, and point its mail at a folder. Fastmail warns that an alias can take up to 15 minutes to start working, so a signup form that rejects it immediately is worth retrying rather than debugging.

Masked Email is under Settings, then Privacy & Security, then Masked Email. There is nothing to name. You press the button, an address appears, and you paste it.

Masked Email also has an API, which is what third-party apps including this one use to list addresses and block one without opening settings. Masked Email Manager does not touch aliases: the token it holds is scoped to Masked Email addresses.

Plus addressing, and the subdomain form

you+shop@example.com costs nothing and needs no setup. It arrives in your inbox with the suffix intact, so you can file it with a rule and see which service handed your address on.

However, plus addressing does not hide the address underneath. Whatever you built it on, your main address or an alias, is sitting in the string, and anyone who wants that address can delete the +shop. There is also nothing to switch off afterwards, because you never registered anything: once a sender has you+shop@example.com they can reach you until you write a rule to stop them. Some signup forms reject the + outright as well.

Subdomain addressing, shop@you.example.com, gets past the forms that reject a plus sign and behaves the same way otherwise: the address underneath is still recoverable from it. That address is always an alias here, because subdomain addressing needs one with no dot or underscore in it. Use either form for sorting your mail, not for hiding from a sender you might want to cut off.

Which to use

Use an alias for permanent addresses you share publicly or say out loud, such as billing contacts, personal correspondence, or business roles like invoices@your.company. These should be memorable, and since you rarely retire one, the bounce on the way out costs you little.

Use Masked Email for web signups, forums, trials, and retail checkouts where a service might leak your address. You can shut an address down silently at any time without affecting other services.

Nothing stops you running both. They answer different questions.