Definition
Plus addressing, also called sub-addressing, lets a person create variations of their address by adding a plus sign and a tag to the local part. Mail sent to [email protected] lands in the same inbox as [email protected].
Gmail supports it, and so do many other mail providers, including Microsoft 365 and Outlook.com. Some providers use a different separator, such as a hyphen. Whether it works for a company domain depends on how its mail server is configured.
People use tags to filter mail into folders and to see which company shared or leaked their address.
Gmail also ignores dots in the local part of consumer addresses, so [email protected] reaches the same inbox as [email protected]. That is a Gmail behaviour, not a general rule. On most other domains, dots matter.
Why it matters
The plus sign is a valid character in the local part. Forms that reject it turn away real users, often the privacy-conscious ones who are careful about who gets their address.
Plus tags also create duplicates that are hard to spot. One person can sign up three times with three different tags. If that matters, for example to stop repeated free-trial sign-ups, normalise addresses for comparison by stripping the tag, but keep the original for sending, since that is what the person chose.
A plus-tagged address is not disposable. It points at a real, long-lived inbox and should not be treated like a throwaway address.
How SimpleCold handles it
SimpleCold accepts plus-tagged addresses as valid syntax and verifies them like any other address. They are not flagged as disposable. The result tells you whether the mailbox accepts mail, with the usual category and reason.
Check how a specific address parses with the free email syntax checker.