How plus addressing works
RFC 5233 describes this as subaddressing: everything after a + in the local part of an address (up to the @) is treated by supporting mail providers as a tag, ignored for routing purposes — jane+shopping@gmail.com and jane+newsletter@gmail.com both deliver to jane@gmail.com’s inbox, while the tag remains visible in the recipient field for filtering rules.
Why plus addressing matters for list quality
Someone can sign up for the same list multiple times using different tags on the same underlying mailbox (user+promo1@, user+promo2@), which looks like multiple unique contacts in a naive dedup process but is really one person and one mailbox. Conversely, aggressively stripping tags to “deduplicate” can incorrectly merge genuinely distinct sign-ups if a user deliberately uses tags to separate different accounts or roles.
Plus addressing as a disposable-adjacent tactic
Some privacy-conscious users deliberately use a unique plus-tag per signup specifically so they can identify (and then filter or block) exactly which company leaked or sold their address if spam starts arriving at a particular tag. This isn’t the same as a disposable address — the mailbox is real and permanent — but it does mean the practical effect for senders is similar: an address that can be traced to and selectively cut off from a single source.
Why verification should treat the full address literally
Because not every provider supports subaddressing, and because the tag is semantically meaningful to the end user (filtering, tracking), verification should check the complete address exactly as submitted, including any plus-tag, rather than assuming it can be stripped — doing so can produce an SMTP response that doesn’t match the real-world deliverability of the original, untagged address.