What RFC would I refer to in order to find out if a leading underscore in the local part of an email address is acceptable?

Example: _johndoe@hotmail.com

Of course, this question can also apply to any leading special character.

I found this line in Wikipedia, but couldn't find any reference within the RFC itself. Did I miss something here?

Local part The local-part of the email address may use any of these ASCII characters RFC 5322 Section 3.2.3:

email-validation rfc
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