My Gmail account setting is set to "Enable POP for all mail". I have an SMTP client that sends emails to my Gmail account. I also have an POP client that is able to pop emails from my account. In order for me to pop a new message sent using my SMTP client, I have to manually reset my Gmail account setting to "Enable POP for all mail" again. Can anybody tell what the problem might be?

Also, is it possible to change Gmail settings using an implemented program (in C#)?

I only know yu have to allow gmail alternate email port

Thanks Juan for your input. I have modified my code to now enabled my SMTP client to send mail through both ports (i.e., 465 and 587), but the problem still persist:
For each message that I send using my SMTP client, I am not able to pop it (with my POP3 client) without having to manually reset my Gmail account setting to "Enable POP for all mail".

HEEEEEEEEELLLLLLLLLLPPPPPPP!!!!!!!!!

Try using port 995 on pop.gmail.com