Recently I’ve heard from various different clients having problems when sending emails to the NHS
The error message usually reads
<****.com #4.0.0 smtp;451 ******@****.nhs.uk>… Requested mail action not taken: mailbox unavailable
Due to the message specifically saying “mailbox unavailable” and it’s several different clients all sending to the same organisation I’ve been placing the blame with the NHS
This morning I found out that assumption was wrong
I was doing some work on one of our clients servers and needed to restart the SMTP service
About two minutes later I got a call from the client in question to say they had all suddenly started get a glut of NDR’s
Initially it looked like they were all for messages sent to the NHS but we found one or two that were to different domains and also spotted that some of these messages were originally sent last week
It was one of the other emails that made me look into this further. The error in this case was
<****.com #4.0.0 smtp;450 <****@******.com>: Recipient address rejected: Greylisted for 5 minutes>
Greylisting!
This is where a mail server temporarily rejects a message when it is first sent. When the mail server retries it will be accepted on the second attempt. The theory is that spammers get so many rejections they don’t try again (more info available at http://www.greylisting.org/ and http://en.wikipedia.org/wiki/Greylisting)
While looking into this previously I’d been told that the NHS were using greylisting as part of their spam prevention measures
Looking at the two error messages that both use the same SMTP failure code (4.0.0) which indicates a temporarily failure (i.e please try again!)
The NHS description is less helpful than the other one
So i still had to figure out what had caused all the NDR’s to suddenly appear
On the greylisting.org site there is a page called “Problem MTA’s”. This a single page with one entry
Microsoft Exchange 2003 and greylisting
There is a bug in MS Exchange 2003 when sending to greylisting servers. Sometimes the server will fail to re-queue messages sent to some servers that implement greylisting. More information:
groups.google.com/group/mi…64d5749ee7cb
groups.google.com/group/mi…07ac14b116db
According to ozinm on our forums there is a possible hotfix available from Microsoft now: www.greylisting.org/forums/showthread.php?tid=18
The threads there talked about the exact same problem I had. It also looks like the problem has been around October 2005. It has various workarounds including using a batch file to restart the SMTP service every day!
Eventually a hotfix mentioned is here: 934709
HOWEVER! This is for servers that are running Windows 2003 with the SMTP service. It doesn’t apply if your using Exchange 2000 / 2003
After a bit more searching I finally found this
This fits the problem exactly. I’ve applied to the server I’ve been dealing with today so I’ll keep an eye on it and apply to the other servers that have noticed similar issues
Another solution would be to use a smart host to deliver your messages for you
One thing i have learnt here is that I probably should have double checked the SMTP code instead of relying on the error description. I generally only get to the SMTP code if I can’t find a problem using other troubleshooting methods (Exchange messages tracking, telnet tests, etc)
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I’ve a problem like this with Exchange 2003 SP2 where emailing to *.scot.nhs.uk has the greylisting error. However my company uses a disclaimer solution called DisclaimIT and it for some reason after the 451 ******@****.nhs.uk>… Requested mail action not taken: mailbox unavailable
Exchange drops the message and resends without using any of the Disclaimer software so we get emails without signatures.
According to the software developer of DisclaimIT it’s another big bug in Exchange 2003.
In our case an email is sent to 4 different servers linked to *.scot.nhs.uk before being accepted.