This setting should be rarely needed, and should be normally left blank. It is here for historic purposes.
If you put some text in this box, then VPOP3 will refuse any SMTP connections from clients whose 'HELO'/'EHLO' command data contains the text specified here as a substring. Eg, if this setting is set to '.myisp.com', then VPOP3 would refuse SMTP connections from any client which sends the command 'HELO box23.myisp.com'
Delivery Status Notifications are an SMTP extension (RFC 3461) which allow customised delivery status notifications in a more controlled/managed way that 'Return Receipts'.
If you check this box, then VPOP3's DSN support is disabled. There is normally no reason to do this, but the option is here if you wish.
According to the standards, the '%' character is allowed in the 'local part' of email addresses (the part before the @ symbol). However, it is rarely used in practice.
In the 'old' days, using the percent symbol in an email address had a common use which quickly became abused when spam started being created. You used to be able to send a message to something like 'firstname.lastname@example.org', and the message would be sent to Apple's mail servers who would strip the @apple.com, and replace the last % with a '@' symbol, and forward the message on. This could be used legitimately for reaching mail servers which may not have very good Internet connectivity, as you could specify a route.
Note that VPOP3 will not interpret the % symbol this way, but spammers will still try to use this trick, so, unless you specifically want to allow % characters in email addresses, turning it off will submit VPOP3 to less load from spammers trying it on. Also, some security scanning software may throw a wobbly if it sees that VPOP3 accepts the % symbol, even though it's actually perfectly safe.
According to the standards, the '!' character is allowed in the 'local part' of email addresses (the part before the @ symbol). However, it is rarely used in practice.
In the 'old' days, some Linux servers would use the '!' ('bang' character) as an indication to run a command with the received email. So, sending a message to '!email@example.com' might make your mail server delete itself…
For obvious reasons this is not widely implemented today, and VPOP3 certainly doesn't interpret the ! symbol this way, but hackers can still try to use it, so turning off VPOP3's support for '!' symbols in email addresses just makes VPOP3 look safer.
If this option is checked, then VPOP3 will automatically add a 'Date:' header field to locally sent messages if it doesn't already exist.
The Date: header field is one of the few mandatory header fields, so all email sending software should automatically add it, but occasionally you may encounter some bespoke email software which doesn't add the header correctly, so you can turn this option on to make VPOP3 add one in that case.
If a message is received using SMTP, then the recipients are specified using an SMTP Envelope which contains the addresses of the sender and recipients. When a mail server, such as VPOP3, delivers the message into a user's mailbox the envelope is discarded as it is of no further use.
In some cases, the mailbox may be accessed by some other software (such as another instance of VPOP3) for delivery to another site with further sorting based on message headers. In this case, BCCd messages can be misdelivered, because the envelope information has been discarded, and the message headers do not contain details of the BCC recipients.
Turning this option on will make VPOP3 add the SMTP envelope data as new lines in the message headers beginning with X-VPOP3-ORIGRCPT. These can then be used by the onward mail sorting software to see who the message recipients were. The downside is that there may be privacy implications as BCCd recipients are now listed in the message headers.
The VPOP3 SMTP service will usually reject unknown local recipients with an error message back to the sender. In most cases this is sufficient as it means that the sender is notified, and the message will not generate error messages later.
However, in some cases, administrators may be interested in this, so you can turn this option on to make VPOP3 log the failed recipients into a 'badsmtprecipients.log' log file, and you can use the 'View Log' button to view the log file.
Minger (Mail pINGER) is a draft protocol used between mail servers to allow authenticated verification of email addresses. This can be useful if one server is forwarding mail onto another server; it can use Minger to check the recipient email address is valid automatically without having to have a complete list of valid addresses maintained on the second server.
As this protocol is authenticated, it can be left safely running, and it will not leak information, or cause any noticeable server load, even if it is not in use. However, you can turn it off if you wish if it is not being used.
The Minger Secret is a 'password' which is shared between the Minger client and the Minger Server.
VPOP3's LAN Forwarding Configuration supports the use of Minger when LAN forwarding wildcarded email addresses to another server.