If enquoted multiple @ are allowed. I have to ask why do you need this information. Please please please do not try to write a regex / function or whatever to validate emailaddresses. You have to worry about 3 rfc’s (and perhaps 4 if you want to take into account unicode). And you will fail and it will drive you mad.
See my previous answer for more information and a regex you may use if you have an older version of PHP
P.S.
In case someone missed my point: do not try to come up with some validation of your own to validate emailaddresses yourself “ever”.