You're able to add multiple email addresses to your Semaphore account. This is done from your Account Settings page.
Each new email you add needs to be verified. Once you do that, the email can be used as an option in project's notification settings. Email adresses are globally unique on Semaphore, so two users cannot add (and verify) the same address.
If a user is subscribed to personal work notifications Semaphore will use his verified email adresses to match git commits with his Semaphore account.
The "Primary email address" is the one you are using to sign in on Semaphore. It is also the default address for email notifications on all new projects.
There can be only one "Primary email address" per account and it can be later changed to any verified email address. Email cannot be deleted while it's used as primary.
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