The best thing to do as far as I know is to read up a bit on Regular Expressions, and try to build up a function that will compare your email-adress(es) with a regular expression using eregi() or ereg() -functions to check the adresses.
you can read more about eregi() and ereg() at php.net, and you can try this adress for an introduction to regular expressions:
http://www.studentsamfunnet.no/Basic_Syntax_Regular_Expressions.pdf
Regular Expressions can seem a bit hard at first, but it's a good thing to know how to write them as they can come in handy for a lot of purposes later in programming.
There is though tough to write a function that will accept and check for every possible email-adress that exist, but a bit of checking with reg.ex will at least filter a lot of input away.
