Problems Sending Emails

Here’s an informational article to help explain why ‘proper’ emails might bounce.

My legitimate email was rejected – why?

When mail servers receive incoming requests, they are now employing a variety of methods to determine if the sender is a spammer. These include;

–    IP realtime blacklists – checking with a database provider such as Spamhaus whether the server that is connecting is on a list of servers that is known to be sending spam

–    IP-block blacklists – checking (usually with a database provider but sometimes using other methods) whether the server that is connecting is connecting from an address that is contained in within a block of addresses known to be used by internet providers as a dynamic range; that is, users of these addresses are not static and in theory should not be running email servers. IP-Block blacklists can also be used to reject email from servers in particular countries, such as far-Eastern countries where a lot of spam comes from.

–    Checking whether the name reported by the connecting server (e.g. matches the IP address it is connecting from – this requires a DNS A-record that correctly resolves to the mail server, and a PTR record for reverse look-up

–    Checking whether the email domain in the ‘from’ field has an SPF record, and if it does, whether the server sending this message is on the list. SPF is a DNS record which says ‘for our domain, only the following servers should be sending mail on our behalf; anyone else is probably just pretending to be us’

–    Greylisting – not accepting the message first-time around, and making the sending server retry later; since spammers rarely bother retrying

–    Content filtering – looking at the content of the email, looking for keywords and phrases, layouts, etc to judge whether the email is likely to be legitimate. These are usually the most difficult to get right, since a subject matter that one company might consider spam, another might deal in buying and selling of such.

– Recipient filtering – looking on a blacklist to see if the email address in the ‘from’ address is known to be sending spam.


When an email is rejected you get an email back telling you that it was undeliverable – and crucially the reason it was undeliverable. Few people ever read these messages in-depth, but they contain the key to the reason your email was rejected. The difficulty is that there is no one-thing that means you are classified as spam or not, and no standard agreed method of determining legitimacy. Therefore everyone is doing anti-spam differently, so fixing your spam reputation with one company won’t necessarily fix it for everyone.


If you are having difficulty sending emails from your email server, please give us a call, and we’ll ask you to forward the NDR (Non-delivery report) email to us for analysis.