What is an email alias?

Aliases are everywhere in business mail. A department address routing to three people, a personal alias forwarding to a phone, a departed employee's address still reaching their manager.

For SMTP purposes an alias is simply a recipient the server accepts, so a probe returns the same 250 it would for a normal mailbox. That answer is correct and complete.

Forwarding does have a deliverability consequence. The forwarding server relays from its own IP, which breaks SPF, so an aligned DKIM signature is what carries the message through DMARC.

Privacy relay services are the consumer version of the same idea. Apple's Hide My Email generates a working alias per service, and it can be deactivated at any moment without the underlying mailbox changing.

Say your list has maria@acme.example, a buyer who opened everything you sent last year. Maria left Acme in March. Instead of deleting her mailbox, IT converted the address to an alias that forwards to her manager, Tom, so no vendor mail gets lost.

From outside, nothing changed. The server answers 250 to the recipient, so verification says valid, and it's right: mail to that address is delivered. Your platform shows opens and even the odd click, because Tom skims what arrives. According to your records, Maria is an engaged contact. In reality you're mailing a person who never signed up, under someone else's name.

The signals that give it away are human ones. A reply signed by somebody else. An unsubscribe that arrives right after a long engaged streak. A LinkedIn profile showing a new employer. No protocol check will surface this, which is why job-change data and reply handling matter for B2B lists alongside verification.

Forwarding produces one of the more confusing bounce reports you'll meet. Say info@smallfirm.example forwards to the owner's personal mailbox at another provider, and that mailbox is full. Your message is accepted by the first server, forwarded, and refused by the second. Depending on how the forwarder handles the envelope, the failure notice can come back to you naming owner1987@otherprovider.example, an address that appears nowhere in your list.

When a bounce names a stranger, check the Received headers in the attached original, find the hop where the recipient changed, and suppress the address you mailed.

Apple's relay addresses deserve a note as well. Addresses ending in @privaterelay.appleid.com come from Sign in with Apple, and Apple only forwards mail from sender domains you've registered in your developer account. An unregistered sender gets rejected, so a list of those addresses can bounce heavily for a reason that has nothing to do with the mailboxes behind them.