What is a sub-address?
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A sub-address is the tag you add after a plus sign in an email address. It's the "newsletter" in captain+newsletter@deepcurrent.io. Your email still goes to captain@deepcurrent.io, but the tag travels with it.
Why senders care: you can create unlimited tagged addresses from one inbox without configuring anything. Need to track signups from five different landing pages? Use captain+page1@, captain+page2@, etc. All five land in the same inbox, but you can filter or route them based on the tag.
And This works because of plus-addressing, a standard most mailbox providers support. Gmail, Outlook, Fastmail, and iCloud Mail all treat the plus and everything after it as metadata. The message routes to the main address (the part before the plus), but the tag stays in the headers so you can filter on it.
Common use cases: tracking campaign sources ("newsletter" vs "webinar" signups), testing inbox placement (send to yourself+test1@, yourself+test2@, etc.),Segmenting list imports (import+vendor1@, import+vendor2@), creating throwaway addresses for sketchy signups (you can filter or block the tagged version later without losing the main address).
One historical note: some older systems use a hyphen instead of a plus. The qmail mail server popularized captain-newsletter@domain.com, and you'll still see it on legacy setups. Most modern systems default to plus, but both work if the receiving server supports it. What to watch for as a sender: some signup forms reject addresses with a plus sign because developers mistakenly think it's invalid. It's not. If your form validation blocks plus signs, you're blocking a legitimate RFC-compliant address and annoying power users who rely on tagging. Don't be that form.
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