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Re:Firefox Monitor by Moru on Wednesday June 05, 2019 @12:49PM (#20106340)
As does Gmail. But not all services supports the + on login forms.

I believe this was part of the email standard?

Re:Firefox Monitor by StavrosK on Wednesday June 05, 2019 @12:56PM (#20106429)
The email standard does include the +, yes. Here's a talk on the topic:

https://youtu.be/xxX81WmXjPg

But yes, that's why Fastmail supports the alternative syntax.