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Plusnet Email servers giving random SMTP 550 errors for inbound emails

acleach
Hooked
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Registered: ‎14-04-2018

Re: Plusnet Email servers giving random SMTP 550 errors for inbound emails

This. The fact that it's intermittent might make it more difficult to diagnose, but it necessitates a different approach. It's not "Oh! It happened at time T; what was going on then?" but rather "X can't happen, but if it were to, what would make it happen?"

 

It's fairly obviously a DNS issue -- "Domain doesn't exist" -- which can't happen because nothing can fail that would make it happen; but what would need to fail to make it happen? Has that failure occurred?

Townman
Superuser
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Re: Plusnet Email servers giving random SMTP 550 errors for inbound emails

It is one thing to profile a failure, it is something entirely different to identify the cause, especially where the issue cannot be created at will.

The DNS "server" is not a simple construct, it is a distributed resilient responsive "network" of resources … any single component of which having an occasional "cough" could cause this issue.

Superusers are not staff, but they do have a direct line of communication into the business in order to raise issues, concerns and feedback from the community.

Anonymous
Not applicable

Re: Plusnet Email servers giving random SMTP 550 errors for inbound emails

It’s not a DNS issue @acleach, if the email server is RFC compliant then it means that the recipient is unknown at that domain.

acleach
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Re: Plusnet Email servers giving random SMTP 550 errors for inbound emails

@Anonymous,

 

Errors such as this one quoted from an earlier message

 

Action: failed
Final-Recipient: rfc822;xxxxxx@xxxxxx.plus.com
Status: 5.0.0
Remote-MTA: dns; mx.avasin.plus.net
Diagnostic-Code: smtp; 550 #5.1.8 Domain of sender address <xxxx@dtmsoftware.com> does not exist

 

certainly appear to indicate a DNS issue. "Domain does not exist"

 

However, if Plusnet were a bit more forthcoming then we might be reassured that they are actually looking into it. They could at least say what the cause is.

Anonymous
Not applicable

Re: Plusnet Email servers giving random SMTP 550 errors for inbound emails

Fair comment @acleach, I didn't read the entire thread as my comment was based on my understanding of the RFC (821) reasons for the generation of the 550 error code cited in the topic title.

acleach
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Re: Plusnet Email servers giving random SMTP 550 errors for inbound emails

@Townman Yes, it's not simple. I have to deal with "What on earth caused that?!" in the day job too. But not to have any information about investigation or fix after this length of time isn't really defensible, I'm afraid.

I've also had email rejected by Plusnet with this 550-5.1.8 message, which I haven't had from any other recipient. I suppose that might mean that only Plusnet actually do that check: it's a very straightforward anti-spam measure which is probably obsolete given how easy it is to spoof email addresses. The rejection message is also rather pointless: if the sending domain doesn't exist, why send a rejection message there? If it does exist, why not at least customise the message to apologise for the glitch and suggest resending the email?

thunderbird6
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Re: Plusnet Email servers giving random SMTP 550 errors for inbound emails

It was established over a year ago, and many times since, that the messages being bounced come from domains with correct DNS records. The issue is DNS related but has to be a DNS lookup failure within the PlusNet systems. Given the consistent and unique nature of the error I can't see that it should be so difficult to trap it and identify the cause. One is left with the assumption that no urgency or adequate resources are being applied to the problem since most PlusNet customers remain unaware that their legitimate mail is not reaching them, and where they are, they place the blame on the senders and not PlusNet.

gariac
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Re: Plusnet Email servers giving random SMTP 550 errors for inbound emails

I run my own email server and I assure you I know every 550 I issue. You just read the logs. 

 

 

 

Generally you make sure the domain exists (MX record) and there is a reverse pointer at the IP address. I don't require reverse DNS to match since the vast majority of email uses virtual mailboxes, i.e. many domains at one IP.  

 

This is probably an issue with BIND, not the mail server, since many DNS checks work. It could be a timing issue. 

 

 

At least 25% of the email I bounce is due to DNS issues, but not by mistake. 

perl /usr/sbin/pflogsumm -d today /var/log/maillog* | less

 

works quite well to read your logs. 

 

Silanthril
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Re: Plusnet Email servers giving random SMTP 550 errors for inbound emails

This is why I don't understand the length of time it's taking to resolve the issue. I, and no doubt others, have sent email headers identifying the precise moment the email is rejected. I cannot see why it's so hard to go through the log for that time and see what has happened.

 

I am probably vastly over simplifying the solution, but as Plusnet tell us nothing it leads to assumptions being made.

Townman
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Re: Plusnet Email servers giving random SMTP 550 errors for inbound emails

There are real dangers in making assumptions.

In a high availability high resilience infrastructure there can be acres of technology between the requestor and the servicing DNS instance.

The mistake people make is presuming everything is simple single instance services with nothing in between and therefore believe all solutions are plainly simple. Even having found a suspect, most stuff is all third party these days - which means getting acceptance by the third party that in their hardware / firmware / software in some (undefined?) circumstances a particular behaviour is seen - who then need to be willing to investigate, resolve and release a fix. Such fixes then needing to be site tested before deployment across the estate.

Proving fixes for a not reproducible at will problem is not the easy task some around here seen to believe. All too often with issues such as this, one can discern the symptoms but not the cause. If anyone thinks this is easy, then I’d question if the really understand big IT?

As someone suggested timing issues can cause odd failures, where essentially there’s nothing actually wrong 99.998% of the time. The infrequency of failure makes detection and resolution much more of a challenge.

Superusers are not staff, but they do have a direct line of communication into the business in order to raise issues, concerns and feedback from the community.

cerij101
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Re: Plusnet Email servers giving random SMTP 550 errors for inbound emails

Surely a year is enough to work through all the intricacies you mention? For a major IT provider anyway. I could understand if it was some kind of free service but people pay good money for this...

Luckily I use Gsuite for email as it is business critical, but i feel sorry for all those average PN customers who don't visit the forums and don't know they are losing email.
Anonymous
Not applicable

Re: Plusnet Email servers giving random SMTP 550 errors for inbound emails


@Townman wrote:
In a high availability high resilience infrastructure there can be acres of technology between the requestor and the servicing DNS instance.

Of course we know you mean instances here as there should be multiple DNS servers in a fault tolerant system. Wink

 

cso
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Re: Plusnet Email servers giving random SMTP 550 errors for inbound emails

@Townman - I'm sure there are some assumptions being made by people (myself included) but you might also find that these assumptions are potentially more correct than wrong.

 

I admit I don't know much about the current email setup at Plusnet (it's probably been 8-9 years since I stopped being as involved as I once was), but I can only make assumptions that it is probably architecturally the same as it used to be when I was far more involved in things... which is multiple servers behind a load balancer, and talking to (probably) other load-balanced servers. 

 

This doesn't change the fact that it's been over a year since it was reported - nor do Plusnet appear to have made any contact with people that are affected (who may also control the domains which are being rejected) to work with them to try and diagnose the issue - that is the point that is more infuriating.

 

It wouldn't surprise me if there is some form of pattern behind the issue, but it just needs someone dedicated to working out what it is and working it out, particularly with some of the affected users or sending domains. As in the past, I'm happy to work with people to try and help track it down if necessary. I strongly suspect my sending domain sending to one of my PN accounts is not going to appear in the logs that often, so someone that was able to analyse them from all the servers might be able to work it out...

Townman
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Re: Plusnet Email servers giving random SMTP 550 errors for inbound emails

I believe that the evidence points to a problem of an intermittent nature - not all emails originating from “domain not known” are treated as such - therefore this is not really a matter of sorting out DNS somethings with the domain owners.

It is much more a matter of understanding why occasionally the system delivers the wrong answer ... but a similar query is then resolved correctly. I am sure that you can understand that there is a country mile difference between seeing what IS happening and knowing WHY it happens.

Knowing the symptoms, monitoring the symptoms does not necessarily lead directly to identifying the cause, even when narrowed down to a component in the solution stack. Even when that’s done, resolution might again not be simple.

There have been a number of DNS odd ball issues over the years which have eluded identification - such as triggering IP address spoofing with TG582n routers which became a headache with Win8 onwards. We could see the consequential symptoms but not the cause - which had to imply a (transient?) DNS lookup failure.

Superusers are not staff, but they do have a direct line of communication into the business in order to raise issues, concerns and feedback from the community.

ihgs
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Re: Plusnet Email servers giving random SMTP 550 errors for inbound emails

If a random intemittent lookup failure is the cause, a workaround could be to retry the lookup (for example, five times at one-second intervals) before reporting this error. The number and frequency of retries could be adjusted as necessary.