The Mail Sender tells VPOP3 how to send mail out to the Internet. If you want to send mail for specific users to another specific mail server (e.g. a local Microsoft Exchange Server or another VPOP3 server) then use the LAN Forwarding feature instead.
VPOP3 can send outgoing mail using two methods:
For most people, this is the option you should choose. This is intended for situations where you will send mail through another mail server (eg your ISP's SMTP mail server). If you have a dial-up, ADSL, cable, satellite connection, then you should almost certainly use this option unless you are very technical and know what you are doing.
Many mail servers will reject mail which is coming directly from a dial-up/ADSL/cable/etc connection, because a large majority of that type of mail is spam, so you should send mail through your ISP's mail server. If your ISP's mail service is so poor that you want to bypass it, then you should consider changing ISP or signing up to a third party mail relay service (we do one for £40 per year - contact email@example.com, or you could look at www.authsmtp.com and others).
See SMTP Relay for more information.
This option should be used when you do not have access to a mail relay server. This usually applies if you have a 'high-end' connection to the Internet, such as a leased line, or a server based at an ISP's datacentre.
With this method, VPOP3 will send mail directly to the recipient's mail server, so it does not need an intermediate server.
To have any chance of this working you will almost certainly need to have a reverse DNS entry configured for your Internet IP address, contact your ISP for help with this.
If you try to use this option with an ADSL/cable/dial-up account you will find that many recipients (especially the large hosts, such as Gmail, Hotmail, Yahoo etc) will reject your mail, and there is nothing you can do about that. These mail servers can tell that you are on a dial-up account from your IP address, and they are configured to reject mail from dial-up accounts. If you still wish to use SMTP Direct for other mail in this situation you will need to tell VPOP3 to route mail for these specific domains through a relay server instead.
See SMTP Direct for more information.