Which of these is an valid email address?

Which of these is an valid email address?

• All of them
• Michael.Scarn@[117.42.15.92]
• "dogs@r.com"@catsrule.com
• Security+@comptia.org 

 

EXPLANATION

Based on RFC 5322,
email address can have two "@" symbols as long as the first one is in quotation marks to identify the local name versus the domain name.  IP addresses can be used to identify the domain name.  The "+" symbol is valid in local names.

SOURCE

https://tools.ietf.org/html/rfc5322