Reading from POP3

[ARFX]

Dear guys

For a project, I must develop a pop3 client application that checks mailbox frequently. POP3 mail box may be Gmail or may be any other SMTP/POP mail box. (Emails are sent from a web site to clients)

I wanted to ask is there any frequency limitation for logging in/requerying mail box, when end user wants to use Gmail?

and which methods are preferred for getting known if a new email is inbox?

Thanks