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When you have a Soverin e-mail account, whether it is a '@soverin' address or an address on your own domain you can use the + syntax for it.
What is the plus (+) syntax or sub-addressing?
You can add an extra part to the e-mail address by making use of the + sign. So an e-mail address like 'email@example.com' is a valid e-mail address. When such an e-mail arrives at the servers of Soverin the e-mail is delivered to the mailbox belonging to firstname.lastname@example.org (as you would expect). However if you have a folder named "mike" in the root of your mailbox (same level as the INBOX), the e-mail will automagically be filed into that folder instead of the INBOX.
What can I use it for?
If you want to track who you hand your contact details to you could hand out e-mail addresses using the + syntax. So if you register at a website you can add for example the website's name to the address to keep track of all the message you receive from that website (or track it if they hand out your e-mail address to someone else).
More information: https://en.wikipedia.org/wiki/Email_address#Subaddressing
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