If you need to give an alternate +Address on forms that may send email you wish to easily sort or know who the sender is based on the address they used to send email to you. (not suggested for use with mailing lists you may need to unsubscribe from later - see fine print below)
Plus Addressing or Subaddressing is defined as a way to support dynamically created recipient (not sender) email addresses for mailboxes. In clearer terms, you can receive email from a plus address, but not send messages from a plus address.
An email address uses the basic syntax: NetworkID@<domain>. For example, NID@wsu.edu
Plus Addressing uses the syntax: NetworkID+<tag>@<domain>. For example, NIDemail@example.com.
The original email address must be valid; the +tag value that you add is arbitrary, although regular character restrictions for email addresses apply (for example, no spaces).
To automatically identify and filter email messages that are sent to plus addresses, use Inbox rules to act on those messages. Using the condition Recipient address includes, you can specify an action for messages sent to a particular plus address, such as moving the messages to a folder.
The fine print:
- Some web forms don't support plus signs in email addresses.
- If you have subscribed to some email list subscription services using an email address, and you need to unsubscribe from them, you must use the email address that you subscribed with. You cannot unsubscribe by sending emails with plus addresses. You also cannot unsubscribe from some email subscriptions using unsubscribe email messages with plus addresses if you have subscribed to them.
The basis for this came from https://docs.microsoft.com/en-us/exchange/recipients-in-exchange-online/plus-addressing-in-exchange-online