What is an alias vs. a mailbox?
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An alias is an alternate email address that forwards mail to a main mailbox without storing anything itself. A mailbox stores mail directly and has its own password, storage quota, and login credentials.
The distinction matters more than you'd think. When you send from an alias, the mail is technically coming from the underlying mailbox's infrastructure. That means authentication records (SPF, DKIM, DMARC) need to be configured for the domain of the alias, not just the mailbox. If you set up support@yourcompany.com as an alias pointing to john@yourcompany.com, and someone sends from the alias, mailbox providers will check authentication against yourcompany.com. If the records aren't set up correctly, the mail can fail authentication even though John's personal mailbox works fine.
Aliases are useful for organizing incoming mail (all sales@ and info@ messages land in one place), but they don't give you separate sending identities. If you need distinct sending reputations for different departments or brands, you need separate mailboxes or separate domains entirely.
Most modern systems support plus addressing (also called sub-addressing), which acts like a built-in alias system. You can send mail to captain+shipping@deepcurrent.io or captain+invoices@deepcurrent.io, and it all arrives in captain@deepcurrent.io. The part after the + acts as a tag for filtering. Gmail, Fastmail, and most ESPs support this natively. It's a quick way to track where signups came from or set up automatic filters without creating formal aliases.
What to check if you're using aliases for sending: Make sure your SPF and DKIM records cover the alias domain. Verify that replies to the alias go where you expect. And if you're sending marketing email from an alias that forwards to a personal mailbox, know that complaints and bounces will affect the underlying mailbox's reputation. For high-volume sending, use a dedicated mailbox instead.
If you're not sure whether your aliases are authenticated correctly, run a test send through our Email Header Analyzer to see what authentication results show up.
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