HalBert42 wrote:When the pop3 client picks up the mail, it deletes it from the server. Which should be alright?
Yes. Mercury POP3 Client is somehow equivalent to Thunderbird Email Client - its used for picking up Emails from a POP3 server (in your case there is a POP3 server at domain.com).
HalBert42 wrote:How can the SMTP Client(Mercury) retrieve the email from the pop3 server(Mercury).
It cannot. The SMTP Relay(!) Client (there is no "SMTP Client", its a Relay Client) acts like a local SMTP server, it listens on Port 25 and gets the email from a local sendmail program and "relays" it to a public SMTP Server (like GMX or GMAIL etc.), which finally sends it to the email address. The Relay Client cannot pick up mails from a POP3 server.