Plus addressing explained

Plus addressing, also called subaddressing, lets a person add a tag to their address, as in jane+news@example.com, and still receive the mail at jane@example.com. Gmail, Outlook.com, Microsoft 365 and Fastmail document it. Yahoo and Apple document different features: hyphenated disposable addresses and random forwarding addresses. For senders the rules are short: accept the + in every form, mail the address exactly as the person gave it, and deduplicate with a separate comparison key, never by rewriting the address.

How subaddressing works

The reference is RFC 5233, the subaddress extension to the Sieve mail filtering language. Its introduction describes subaddressing as adding detail information to the local part of an address, split from the original local part by a separator such as +. It calls the two parts the user and the detail:

ken+sieve@example.org
\_/ \___/
user detail

In the RFC's own examples, mail to ken+sieve@example.org lands in a mailbox called sieve that belongs to the user ken, and 5551212#123@example.com reaches voice mailbox 123 at a phone number. The RFC also shows a scheme where the detail comes first and -- is the separator.

Two points in RFC 5233, section 4, matter to anyone who handles other people's addresses. The way detail is encoded is specific to each site or implementation, so splitting addresses from other domains may give wrong results. And when the separator occurs more than once, how the address is split is up to the mail system. In other words, the + means something only to the domain that receives the mail. To everyone else it is an ordinary character, valid in any local part (email address syntax).

What the providers document

Each row comes from the provider's own help page, listed under Sources.

Gmail and the dots rule

Gmail's help page on sending from an alias gives janedoe+school@gmail.com and janedoe+notes@gmail.com as addresses that all go to janedoe@gmail.com, and suggests a filter for each. A second page, Dots don't matter in Gmail addresses, explains that john.smith@gmail.com and j.o.h.n.s.m.i.t.h@gmail.com reach the owner of johnsmith@gmail.com, and that nobody can register a dotted variant as a separate account. The same page warns that this does not apply to work, school or other organization accounts, where dots do change the address. Google's Workspace Learning Center describes plus variations for Workspace mailboxes.

Microsoft: two products, two pages

For Outlook.com, Microsoft's page on sending from a different address describes adding +sitename to your address so you can tell who mail came from, and notes that you cannot sign in or send with it. For business mailboxes, Plus Addressing in Exchange Online says the feature is on by default and that an admin can turn it off in the Exchange admin center or with PowerShell. Exchange first tries to match the full address, tag included, to a known mailbox, and only if that fails tries again without the tag. A + address can therefore be a separate mailbox.

Fastmail

Fastmail's page on plus addressing and subdomain addressing supports both alias+whateveryoulike@domain.tld and whateveryoulike@alias.domain.tld. When the tag matches a folder name, the message is filed in that folder instead of the inbox. Sending from such an address requires setting it up in the account.

Yahoo and Apple: different features

Yahoo's help page on disposable email addresses describes a nickname and a keyword joined by a hyphen, nickname-keyword@yahoo.com. The feature is for Yahoo Mail Plus subscribers (with a free trial of up to 3 addresses in Mail Plus locales), and mail to these addresses comes to the main inbox. Because the nickname is chosen once and the hyphen is an ordinary character, you cannot tell such an address from any other hyphenated address.

Apple's Hide My Email for iCloud+ subscribers creates unique, random addresses rather than tags on a real one. Mail is forwarded to the person's own account, replies keep that account private, and the person can deactivate or delete an address at any time. Nothing in the address links it to the person's main address, and it is not meant to.

Why people use tags

• Sorting. Gmail filters, Outlook inbox rules and Fastmail folders can all act on the tag.
• Tracing. Microsoft suggests a tag per site, so mail from anyone else to that address shows the site shared it.
• Control. Yahoo suggests deleting a disposable address that gets too much spam, and Apple lets people deactivate or delete Hide My Email addresses they no longer use.

All three are reasons a person gave you that exact address. Treat it as their choice.

What senders should do

Accept the plus sign in every form

+ is part of the standard character set for local parts, and Microsoft notes that the tag follows the usual SMTP character rules. A form that rejects it turns away real people who typed a correct address. The HTML valid email address definition accepts it, and so should your server-side check (signup form validation).

Never strip the tag before sending

Removing +news from jane+news@example.com looks harmless, but it changes the recipient. RFC 5233 says the encoding is site-specific, and Exchange shows that a full tagged address can belong to a separate mailbox. On a domain without subaddressing, jane+news and jane can be two different people, or the stripped address may not exist and bounce. Even where both reach the same inbox, the person's filters and their way of tracing leaks depend on the tag. Store and mail the address as given.

Deduplicate with a comparison key

To limit free trials to one per person or to merge duplicate contacts, compute a second field for comparison and keep the real address untouched:

1. Lowercase the domain.
2. Only for domains whose rules you have confirmed, remove the tag: for example, for gmail.com, the + and the tag after it, and the dots in the local part.
3. Leave every other domain alone. Do not remove dots for Google Workspace domains, where they count, and do not treat a hyphen as a separator.
4. Compare keys; send to the stored address.

Two addresses with the same key usually reach the same person. Two different keys do not prove two different people: Hide My Email exists so that they cannot be linked. See list cleaning for duplicate handling in a list.

Respect the privacy signal

A tag often names the place the person gave the address, which is the point of Microsoft's tracing advice. If you share or sell a list, tagged addresses show it at once. Collect addresses with consent, ideally confirmed by double opt-in, and never invent tagged or "cleaned" versions of an address the person did not give you. Private addresses can also stop working when the person deactivates or deletes them, so process bounces and suppress the address when one comes back.

How our results treat aliases

Send the address to the API as the person typed it. A tagged address is a normal address and gets one of the usual statuses. Two reasons of the valid status relate to this topic, with the meanings our catalog gives them:

• alias: the address is an alias that forwards to another mailbox, and mail sent to it is delivered. The action is send.
• normalized: we corrected a harmless formatting detail in the address, and the corrected address accepts mail. The action is send.

Neither reason asks you to replace the address the person gave you. Throwaway inbox services are a separate matter: they get the disposable status, covered in disposable email addresses.

Questions

Does every provider support plus addressing?

No. It depends on the receiving domain. Gmail, Outlook.com, Microsoft 365 and Fastmail document it; Microsoft 365 admins can turn it off; Yahoo's help pages describe hyphenated disposable addresses instead. On a domain without it, a tagged address may bounce.

Should I block plus addresses to stop sign-up abuse?

Blocking turns away real customers. Use a comparison key for one-per-person rules, and rate limits and confirmation for bots.

Can someone reply from a plus address?

Usually not by default. Microsoft says Outlook.com and Exchange Online users cannot send from plus addresses, and Fastmail requires the address to be set up first. Expect replies and support mail from the base address, and match them to the account with your comparison key.

Try it

Check a tagged address in the free email checker to see its status and reason, or verify a full list with bulk email verification.

Related

• The valid status: alias and normalized.
• Glossary: alias address.
• Signup form validation.
• Statuses and actions.

Related guides

• Email address syntax explained
• Disposable email addresses explained
• How to clean an email list
• Double opt-in (confirmed opt-in)
• Email typos and did you mean suggestions
• All guides

Sources

• RFC Editor: RFC 5233, Sieve Email Filtering: Subaddress Extension: user and detail parts, separators, site-specific encoding (sections 1 and 4).
• Gmail Help: Send emails from a different address or alias: plus suffixes delivered to the base address, filters.
• Gmail Help: Dots don't matter in Gmail addresses: dots ignored in personal accounts, but not in work or school accounts.
• Google Workspace Learning Center: Tips to optimize your Gmail inbox: plus variations for Workspace mailboxes.
• Microsoft Support: Send email from a different address in Outlook.com: +sitename addresses, no sign-in or sending with them.
• Microsoft Learn: Plus Addressing in Exchange Online: default on, how resolution works, how to turn it off, no sending from plus addresses.
• Fastmail Help: Plus addressing and subdomain addressing: both forms, folder filing, sending setup.
• Yahoo Help: Disposable email addresses in New Yahoo Mail: format, eligibility, limits and deletion.
• Apple Support: Hide My Email and iCloud+: random forwarding addresses, replies, deactivation.

How we wrote this: drafted by the email-validation.org team with the help of AI writing tools, from our product documentation and the sources linked in the text.