Apple Mail can put an address in the From line that is not the account the message actually travels through. It needs telling how, and the settings sit somewhere most people never open.
Two jobs get confused here. Answering a message that reached you through a forwarding alias, and starting a fresh conversation as that alias. Only the second needs anything configuring, and the first is the one nearly everybody is really trying to do.
This guide covers the second job on a Mac: what to have ready, where the settings sit, and what to check when it fails. Apple renamed Preferences to Settings in recent versions of macOS, so the paths below are written as Apple's own documentation has them at the time of writing.
Start here, because you may not need any of this
If a forwarding service is delivering mail to your Mac, replying to that mail needs nothing configured at all. A service that rewrites the reply path has already done the work. The copy in your inbox carries a reply address belonging to the service, so pressing Reply sends it back out through the service, which relays it on with the alias as the visible sender. No account, no ports, no password, and replying without revealing your address covers the mechanics properly.
One exception, and it is worth reading twice. Reply All. Only the sender's address was ever rewritten. Everybody else on the thread sits in Mail as an ordinary address, so your reply reaches them straight from your real account and shows your real address on the way. On a thread where the masking matters, reply to the sender alone.
So everything below is for one case: writing to somebody who has never written to you. If you never do that from an alias, you can stop here.
What to have in front of you before you start
Your provider publishes all of these. Type them rather than guess, since each is a common cause of a failed send.
- The outgoing hostname the provider gives you, which is not your mail provider's general server and not iCloud's.
- The port. 587 is the submission port, reserved for exactly that by RFC 6409, and it pairs with STARTTLS. Port 465 with implicit TLS is the usual alternative.
- The username the provider issued for this alias. Often the full alias address, sometimes a separate account name.
- The password the provider issued for this alias, usually generated in its dashboard and not the password you log into its website with.
- The alias address itself, character for character. The From line has to match it.
RFC 6409 also requires a submission server to refuse mail by default on a session that has not authenticated, so leave the authentication box ticked whatever any older guide says.
The walkthrough, in Mail's Settings
There are two routes and they suit different providers.
The full account route. In Mail, choose Mail then Add Account, enter the address or choose the account's domain from the list, then follow the instructions. That gives you a mailbox and a sending identity in one go, and it is right when your provider offers IMAP as well as submission.
The outgoing server route. Choose Mail then Settings, click Accounts, select an account on the left, then click Server Settings. Click the outgoing Account pop-up menu and choose Edit SMTP Server List. That window holds every outgoing server Mail knows about, each with Description, User Name, Password and Host Name. Deselect Automatically manage connection settings and the port, TLS and authentication fields appear, which is where your provider's values go.
The catch that stops people is the incoming side. Mail is built around accounts that have a mailbox, so the account route asks for an incoming server even when all you want to do is send, and plenty of forwarding services have no mailbox to offer. If yours publishes IMAP details, take the account route. If not, Mail may refuse to finish, and adding the outgoing server alone will not put the alias into the From menu.
Mail builds that menu from addresses attached to accounts it already knows about. If the alias never appears there, nothing you do in the SMTP list will change it, and the provider's own compose window is the shorter road. The same wall exists in Outlook and, for its own reasons, in Gmail.
Choosing the right From address when you compose
Move the pointer over the From field in a new message, click the pop-up menu that appears, then choose the address. That is the whole operation, and it applies to that message only.
For a default, choose Mail then Settings, click Composing, then set Send new messages from. Mail can also pick the best account automatically, based on the first recipient's address and the mailbox you are looking at. That is convenient, and it is also how a message leaves from the wrong address without anyone noticing. Pin the default and check the From line before sending.
Then send a test message to an address you control and read the raw headers. The From line should show the alias.
How this differs from Apple's Hide My Email
Hide My Email needs none of the above, and where it fits it is far less work. Apple describes it plainly: unique, random addresses that forward to your personal email account, meaning your iCloud Mail address or an address associated with your Apple Account. When you reply, your personal address stays private and the message appears to come from the Hide My Email address.
It needs no SMTP because there is nothing to bridge. Apple does the sending itself: it documents that you can send and reply with those unique addresses in Mail on iCloud.com, so no mail app has to be told anything. The catch is the other side of that. A mask can forward to a non-Apple mailbox, but sending as the mask happens inside Apple's own mail, not from wherever the forwarded copy lands. Apple documents the sending side in Mail on iCloud.com: compose, add a recipient, choose Hide My Email from the From pop-up menu, then send. One published limit matters, since Apple states such messages reach a single recipient only.
On cost, the honest framing is that it comes with iCloud+ rather than free. Apple states you can use it if you, or someone in your Family Sharing group, has an iCloud+ subscription. If you already pay for iCloud storage and want a handful of masks for signups, that is the cheaper answer and this guide is the long way round. What it will not do is put those addresses on a domain you own, which is what lets you change providers later without reissuing every address.
Troubleshooting
Authentication failures. A 535 response almost always means one of three things: the username is in the wrong form, you used the website password where a generated one was required, or the password never saved. Re-enter it in Edit SMTP Server List rather than assume it stuck.
Ports and TLS. These come in pairs, 587 with STARTTLS or 465 with implicit TLS. RFC 8314 recommends implementing both and notes no significant difference between their security properties when the implementations are correct. Mixing them produces a connection that stalls rather than an error you can read, so if Mail sits there on send with nothing to say, try the other pairing before touching the credentials. Port 25 is for relay between servers and is blocked on most home and cloud networks.
Sends, but lands in spam. Send through the provider's own domain and alignment is the provider's problem. From a domain you own, three records have to agree: SPF listing the host that sends on your behalf, DKIM signing with the key the provider issues, and DMARC, which means nothing until the first two align.
One case looks like this and is not. If mail forwarded to you lands in spam, your outgoing settings are irrelevant, because forwarding is an inbound path and breaks SPF unless the forwarder rewrites the envelope sender. That is a question for the forwarding service, and a filter rule is the usual short-term fix.
iOS Mail is the same idea by a different path
The fields are identical, the menu is not. On iPhone and iPad you add mail accounts from Settings rather than from within Mail, and when the provider is not in Apple's list you tap Add Other Account, then Mail Account, and type the hostname, port and credentials yourself. Apple has moved where app settings live between recent iOS releases, so follow its current page rather than a forum screenshot. An account added on one device does not travel to the other.