How to write testcase for Email id ?

ECP:Equlence class partetioning:
its is one of the famous blackbox tech which is used for creation of the test cases.Equelence class is a subset of data that represents the larger calss.

ex:from the same question of validating the login id,if the req says that the mail id must consists of only alphabets then u need to test
1.weather login id accepts numbers.(this is one equlence class)
2.weather login id accepts a spl symbol(this is other equilence class)

Equlence partetioning is testing for equlence class rather than taking exaustive testing i.e in first case u need not test for all the combinations of numbers in the second case u need not test for all the combinations of spl characters.

BVA:Boundary Value analysis:it is also another famous blackbox testcase design tec,where we need to focus on the i/p and o/p boundaries of the system funtion.

ex:in the same login id example if the req damands that the mail id must be 8 chars long we write test cases to check for 7 and 9 chars respectively.

coming to the test case on email id,

1.Check for ECP i.e weather the system accepts all the numbers or alphabets or spl charteristics.here we had derived 3 Equalence classes.
2.check for Boundary value analysis i.e if the req says the login id must be 8 chars long (excluding domain name,ex:xxxxxxxx.gmail.com)then check the loginid for 7 chars and 9 chrars.
3.Check weather the loginid accepts the user name starting with caps letter or number or spl charaters.
4.the spl characters used generally are '_',"." aprt from this the system automatically gives u the '@"symbol.
5.check weather the system accepts
a.more than one spl symbol..i.e xxx.xx_x@gmail.com/xxx@x.xx@gmail.com
b.The same spl characters for more than once i.e xxx_xx_x@gmail.com
6.Check for comination of equelnce classes.
i.e combining alphabets and numbers..i.e aaa2aaa1@gmail.com
7.check the mail can be of same domain name i.e gmail@gmail.com
8..apart from this u can even test the GUI of the login field,when we move the curser on to text field the arrow pointer of mouse must become "I" shape(part of cursery testing),
the curser must be blinking in the testfield so as to make the user to pay attention,and even in some aplications the textfield where the cursor blinks will be in highlighted.

Fuente: http://ssoftwaretesting.blogspot.mx/2010/07/how-to-write-testcase-for-email-id.html