Just to clarify:
- MercuryS: SMTP Server module that receives incoming SMTP email from SMTP clients and from other SMTP servers.
- MercuryC or MercuryE: SMTP Client module that sends outgoing SMTP email to other SMTP servers; MercuryC is a relay client that simply relays all outgoing mail to a specified SMTP server whereas MercuryE is an End-to-end delivery client that can do domain name resolution in order to contact the recipient's SMTP mail server and deliver the SMTP email message directly.
- MercuryP: POP3 Server module provides new emails to POP3 clients
- MercuryD: POP3 Client module downloads new emails from other POP3 hosts on behalf of your users
I recommend that you keep the "do not permit SMTP relaying of non-local mail" option enabled at all times. Otherwise, your Mercury server will become known as an "open relay", which is a bad thing and which will get your Mercury server's IP address added to various blacklists. Instead, I recommend adding your local subnet(s) to MercuryS's connection control's Allow column and optionally enabling the "strict relaying" option. If you have any remote users that use public IP addresses outside of your control, then see about using SMTP authentication in Outlook when relaying email through Mercury.