I hope someone can help, and that I've chosen the relevant Stack Exchange site for my question.

I'm having a strange issue with emails not being received by some people. I have a website that sends out emails, and I've never had a problem with it. I'm BCC'd in on every email that gets sent to I know they are being sent and I'm receiving them myself, however some people have reported that they are not receiving the emails for some reason. These are users that have always received them without issue in the past, and I know they've been sent because I have a copy of the email. They insist that no filtering happens and that they are not going to their Junk folders. Strangely, I can email the users from my email client without issue but if I try and forward the missing mail it doesn't seem to get through to them either. I've checked against blacklists and not had any negatives returned. It is happening for one user both on his office email account and on his personal email account.

My question is this: what else can I do to diagnose this? I am at a complete loss, I'm insisting it's not something at my end as I'm getting the emails through but I'm starting to wonder if that's true as three people have reported the issue today, and they've never had an issue before. This isn't affecting everyone, but three of seven emails so far today have not been received by the users. The website has been running successfully for well over a year now.

Any and all advice would be greatly appreciated!


  • Are you using a third party web application to send emails? What did you check against the black list? I assume the website. – Dave Sep 26 '13 at 10:43
  • Hi Dave, thanks for the response. These particular emails are generated by the 'mail();' php function, pending an update to PHPMailer. If that's what's causing it then I'll prioritise that update, but I've never had a problem before. Also, one of the users has reported that a later email has come through but there appears to be a "dead spot" of about 4 hours where they didn't get through. – JoeP Sep 26 '13 at 10:52
  • Is this via your server or a shared server? – Dave Sep 26 '13 at 11:19
  • A shared server. – JoeP Sep 26 '13 at 11:27

Ideally, you need to check the logs if possible to see what is going on. This will give you a complete break down of each 'stage' of the email process and will hopefully provide extra information.

It also sounds like you're using a third party to send the email, if this is the case, then there is nothing we can do as it could be a bug in their system, for example.

Since your comments mentions it could be just delayed, read up on grey listing

Greylisting (or graylisting) is a method of defending e-mail users against spam. A mail transfer agent (MTA) using greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate the originating server will, after a delay, try again and, if sufficient time has elapsed, the email will be accepted.

And most intersetingly

Greylisting is effective because many mass email tools used by spammers do not queue and reattempt mail delivery as is normal for a regular Mail Transport Agent

  • My comments are a little mis-leading I think, the original missing emails have not arrived at all. It is only ones that have been generated later on that have made it through to users that were having problems before. The part that I find most confusing is that I'm unable to forward the missing emails to them using my own email address (unrelated to the server) but I can send them unrelated emails without issue. This "greylisting" does certainly seem like a possibility though, if I had a higher rep I'd up-vote. – JoeP Sep 26 '13 at 11:32
  • @JoeP But, this only occurs to some recipients doesn't it? In other words, some receive the email, some do not? – Dave Sep 26 '13 at 11:34
  • Correct, but one user that didn't receive an earlier email has received an email that was generated a little later on. – JoeP Sep 26 '13 at 11:40
  • If any of these missing emails turn up I'll be marking this answer as correct by the way, I'm still investigating at the minute though. – JoeP Sep 26 '13 at 11:41
  • 1
    This sounds like some of your clients ISP are blocking this type of email (probably due to content), probably because it is seen as spam... I would be careful of this, too much and you'll get black listed. Please note, many ISP's will share spam filter technology, and so if 1 has made an update it will/may affect multiple ISP's. So, this, coupled with grelisting may be the overall issue. – Dave Sep 26 '13 at 11:42

You can do some manual troubleshooting as follows:

You can find out what the server should be for any domain by looking up the "MX records"

For example here: http://mxtoolbox.com/

The MX record is what your email server looks for to find out where they should send your email.

You can then initiate a manual connection to the server listed in the MX record to see if it is accepting email and what error messages you might get. Use a telnet program like Putty: http://www.putty.org/ and telnet to the email server on port 25. Some of the commands you will need are listed here: http://www.yuki-onna.co.uk/email/smtp.html

So now you can connect to their mail server and send an email using your email address as the "From" address and see how the server responds directly. Try it from your computer and also from the server and see how their email server responds to the different IP addresses. Any email error codes that are returned can be looked up in google or here: http://www.serversmtp.com/en/smtp-error

Once you have checked that you can connect to the server it may tell you why your email is being rejected as spam or for some other reason, but the reason may not be easy to decipher. At this stage I would suggest you ask your customer to contact their support number with the error codes (or lack of them) that you received from their server. You can't log a problem yourself because the problem is on the receiving server. The customer will have to log that problem themselves. If they are not able to get the settings changed on their email server you may be able to find out what you would need to change on your server to get your emails delivered.

Some possible problems are your server being listed on a block list or lacking a SPF record and you can fix that yourself and check using various online validation services. For example https://www.mail-tester.com/

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