The Mail Settings Delivery tab lets you to choose options for how the SMTP Delivery service will deliver email placed in the delivery queue.
Keep copies of sent mail
If you check this box, all processed mail will be copied to the \Mail\Spool\Sent folder under the WinGate install directory once it has been delivered. You should be sure to clear this folder regularly.
Define the length of time, or number of delivery attempts that will be made (which ever expires first) before all queued mail for a domain will be returned to its respective originators.
This section allows you to specify the default method of how WinGate will deliver mail destined for remote locations.
WinGate makes DNS queries (MX lookups) to automatically discover the servers that are prepared to accept delivery of mail for the destination email domains that WinGate has to deliver to. This is the normal setting, and does not require you to use any other server (such as your ISP's) - you will probably want to use this option if you have a full-time connection to the Internet.
If you select this option, WinGate will forward all outbound mail (mail that is not destined for local mailboxes, or otherwise has delivery settings specified in a domain rule configuration) to the mail server you specify here. This will normally be the SMTP server at your ISP.
WinGate tries to deliver directly in the first instance, but if unsuccessful, will pass it on to the defined server for it to try to deliver. This can be useful if your server is running on an IP address that may be blacklisted by other servers (i.e. some dialup addresses are blacklisted by some server administrators).
If you have selected Use Gateway or Use Gateway for undeliverable mail in the How to deliver drop down menu, then this is where you specify the server to be used (you may specify either an IP address or a host name). If you have extra settings that you need to specify, for instance you are delivering to your ISP and they require authentication, you may specify the extra settings by clicking the Select button 
to open the Server properties dialog.
Attempt immediate delivery
If you have this option selected, WinGate will trigger its delivery processing mechanism for a particular message as soon as the message has been received. Otherwise, WinGate will only check for new messages that require delivery as often as you specify in the Process mail every... seconds field.
Reset next scheduled delivery time when new mail for queued domain
Checking this box will set the delivery time of mail already queued, to be sent immediately whenever more mail for that domain joins the queue behind it.
Use secure connection if supported by server (TLS)
If the mail server that the WinGate server is delivering mail to supports TLS (Transport Layer Security) then this connection will utilize TLS.
Enter the maximum number of threads you wish WinGate to use at any one time for delivery of mail. The default setting is appropriate in most cases, however if you are on a heavily loaded machine, you may wish to reduce this number to reduce the workload incurred in WinGate by delivery of email.
Enter how often you want WinGate to check its list of domain delivery jobs to see if one is scheduled for delivery.
The SMTP Delivery service keeps track of mail to be delivered based on files and directories under the \Mail\Spool directory in the WinGate install directory.
WinGate will periodically scan these directories looking for mail that has been received that may require processing.
SMTP Delivery service maintains an internal database which keeps track of scheduled delivery attempts for each domain that has messages pending delivery. Due to occasional delivery problems (for instance a remote server is unavailable) WinGate may sometimes need to reschedule failed delivery attempts.
Parameters associated with the number of tries, the period over which to attempt delivery, and other delivery parameters are configured on this tab.
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