Subaddressing
When subaddressing is true, what plus-addressing and provider aliases mean for signup policy, and how to read the flag from the detect API.
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The subaddressing boolean is true when normalization removed part of the local part—typically a +tag suffix, dot aliases on some providers, or similar alias patterns we recognize.
It does not by itself mean the address is disposable or fraudulent. It tells you the user (or script) supplied an alias of a base mailbox.
Many providers implement subaddressing with a + delimiter:
local+tag@domainRFC 5233 documents this pattern in the context of Sieve mail filtering. Delivery agents treat local+anything as an extension of local when the provider supports it.
EmailGuard sets subaddressing: true when normalization removes that suffix (or equivalent provider-specific aliasing) while building normalized.
The table below illustrates common cases—not a complete list of every provider or alias type EmailGuard handles:
| Pattern | Example input | subaddressing |
|---|---|---|
| Plus tag | [email protected] | true |
| Dot alias | [email protected] | true |
| Provider plus tag | [email protected] | true |
| No alias detected | [email protected] | false |
When subaddressing is true, compare normalized to see the collapsed mailbox. See Email normalization for how deduplication fits into signup flows.
curl -sS \
-H "Authorization: Bearer YOUR_KEY" \
"https://emailguard.co/api/v1/emails/detect?email=checkout%2B2026%40fastmail.com"{
"data": {
"email": "[email protected]",
"syntax_validation": true,
"normalized": "[email protected]",
"subaddressing": true,
"public_domain": true
}
}Common policies:
| Policy | Suggestion |
|---|---|
| Block duplicate accounts | Unique index on normalized, not raw email |
| Allow marketing tags | Accept signup but store subaddressing for analytics |
| Restrict aliases | Reject when subaddressing is true and you need one account per person |
| Combine with disposable check | Alias + disposable domain is a stronger abuse signal than alias alone |
Subaddressing on a legitimate Gmail address is normal user behavior. Pair the flag with disposable, role_address, and your own velocity rules.