how find the port no of a particular email domain

Question

hai'

i am develop a email application that take from add to add and user id password

it work fine only for gmail because i give the gmail email server posrt no to  smtpclient object . but in the text box provided for user he may enter yahoo ,hotmail ..., based on that extension/domain i want to find the port no of the server in .net what can i do how to find the port no. in .net it is web app...

Thanks& Regards,
M.koteswararao.