forwarding, can it loop?

[zoombini]

If for instance, I had a me@ntlworld.com email account & I got it to forward all mail to me2@ntlworld.com which also had forwarding setup to forward all mail to me@ntlworld.com.

Would it make a continual loop that due to the amount of spam being forwarded, the server would grind to a halt?

[Chris W]

urm... what would be the point in doing that... why not just let it go straight to me@ntlworld.com ....

[SMHarman]

There is a mail error* that would bounce this back to the sender stating it is caught in a loop. Don't know the error number, but I saw it recently when a friend of mine screwed up his mail server configurations.

*Assuming the mail server is configured properly

[zoombini]

It was just an "I wonder" thing...

I woudlnt do it, but it was something stupid that got discussed in the office, I'm sure you know what stupid office discussions are like

[gazzae]

http://www.microsoft.com/technet/pro.../frtfytr2.mspx

Looping messages occur when a message moves back and forth between your server and some other (usually external) SMTP server. Sometimes a loop forms when an Exchange user sets up a rule to forward to another account, which has a rule to forward back to the Exchange user. Exchange can usually prevent this type of mail loop by examining the number of Received from stamps in a message's header. When that number reaches the limit you've defined in the SMTP virtual server's Maximum hop count property (on the Delivery tab of the virtual server's Properties dialog box), Exchange refuses to forward the message. (The hop-count default is 15.) Some systems' SMTP gateways, however, keep Exchange from detecting this type of loop because they strip the Received from information out of a message or they generate the forwards such that Exchange sees each one as a new message.

More often, though, message loops occur when an Exchange user sets up a rule to autoforward mail to an external account. The message encounters some external problem (e.g., the external account has exceeded its quota) and generates a nondelivery report (NDR). Because the Exchange account is configured to autoforward new messages, it automatically autoforwards the NDR, which the external account rejects and which causes another NDRââ‚Âà ‚¬ÃƒÂ¢Ã¢â€šÂ¬Ã‚ÂÂ�thus forming a loop. Each time the message makes a round-trip, it grows in size as the systems add new message headers and new NDRs.

External email can also be a culprit of mail loops. For example, an external sender on the way out of the office configures an Out-of-Office ruleââ‚ ¬Ã¢â‚¬ÂÂÂ�and sets the rule to request delivery confirmation. Before the user leaves, he or she sends a message to a user in your organizationâââ€à …¡Ã‚¬Ã¢â‚¬ÂÂÂ�but mistypes the address. Your system returns an NDR, but the Out-of-Office rule automatically replies right back and requests another delivery confirmation. The Out-of-Office functionality might not keep track of replies-per-sender, so it returns a message no matter how many times that message arrives from the same senderââ‚Ã⠀šÃ‚¬ÃƒÂ¢Ã¢â€šÂ¬Ã‚ÂÂ�thus forming another loop.

[Richard M]

I will actually loop a few times (about 5-6 I think) before the headers get too big and it realises something's up.

EDIT: Yeah, what he just posted.

Virtually all SMTP servers have a headers limit - any mail received with more than "n" headers will be rejected. Therefore loops get killed off pretty quickly.