It can be quite difficult for business managers to understand why an email has bounced, and whether this is something that is caused by their equipment or services.
For example, if three of your staff report that customers get bounce-backs when they try to send to your email address, it is easy to assume that this means the problem is at your end, and your email provider or IT provider needs to fix it.
But as they say, "correlation does not equal causation"; just because your business seems to be the common demoninator, does not necessarily mean it is the cause of the problem.
When sending an email, there are three main providers at play, assuming that internet connections at both end are working. There is the mail server of the email sender, the mail server of the receiver, and the spam authorities in the middle. It is this last party which creates a lot of confusion.
Before your mail server will retrieve an email and deliver it to you, it will first check the "reputation" of the sending email server. It does this by referencing third-party spam trackers to see if the address of the sending server is blacklisted for spamming. If the sending server is blacklisted, your email server will drop the message and not deliver it, and the sender will usually get a bounce message describing that the email was not delivered.
With huge providers like Bigpond and Gmail, they have thousands or millions of customers all using their mail servers. A small percentage of those customers will be sending large volumes of emails, and can therefore get flagged for spamming, and the email server's address will get blacklisted until it is manually removed. The larger your provider, the greater the chance of this occuring.
If this is a customer of Bigpond, for example, then suddenly many Bigpond customers using that blacklisted bigpond mail server will start to receive bounce-backs stating that their email was not delivered. Which ones are not delivered will depend upon the recipient's mail server, what spam trackers they subscribe to, and how often they update. So a Bigpond mail server can be effectively blacklisted by some mail servers and not others. This means the user can send to some people but not others.
It also means you may get multiple Bigpond users telling you that they can't email you, and thus it looks like a problem at your end. Meanwhile Bigpond will have other mail servers which are not blacklisted, so some of their customers will be able to email you just fine.
With huge providers like this, they will constantly be getting some server addresses blacklisted due to some of their users' activities, and be removing those listings manually. Thus a person who can't email you because of a blacklisting by their email provider, can sometimes re-send two hours (or a day) later, and the email goes through fine. Sometimes it will just be that their email is routed through a different server, which is not blacklisted.
It is also entirely possible that there IS actually a problem at your mail server, or at the sender's mail server. The way to sort through these three scenarios and diagnose the source of the issue, is to get a full copy of the bounce-back message including the email headers, so that we can trace the path of the email, and see where it was bounced and for what reason.
When no bounce-back information is provided, it can be difficult or impossible to find the source of the issue, until it occurs again and a pattern can be established. All that can be done is to check the correct performance of equipment or of services for which we have administrative access, for example, with test emails sent from and to a variety of sources, and by checking server logs for errors.
Armed with a full set of information including the bouceback message, sender and recipient, and also being able to contact all parties, means that the issue can usually be diagnosed quickly. The problem still may be out of our control to actually fix, but at least we can notify all parties and get the relevant provider working on a fix.