Why are messages not being delivered?

Introduction

Error messages are the best way to determine why something has occurred.

You may be the sending or receiving party, so please read the rest of this document from whichever perspective applies to you.

Causes

The most common reasons for non-delivery follow.

1. The recipient's mailbox is full.
2. The message was too big.
3. The message was marked as Spam.
4. The message was not sent via an authorised server.
5. The sender's email server doesn't have a reverse DNS record.
6. The sender's email server is black-listed.
7. The message is stuck in the sender's Outbox

Explanations

Detailed information, including suggested solutions, follow.

1

Your mailbox may be full

How will you know?

Both the sender and receiver should receive a bounce message, explaining that the message was rejected.

Why?

Most email providers, including zootzone.com limit the amount of email storage. If an inbound message is large enough to exceed the maximum allowed storage space of the receiving mailbox, the server will reject it.

How to fix it

Empty your Trash folder.
Delete messages which you no longer require.
Subscribe to more disk space - send a support request.

All your folders on the server contain messages, so check all of them, especially your Sent folder, which is often as large as your Inbox.

2

The message was flagged as Spam.

How will you know?

Search for the expected message in your Spam folder.

Why?

3

The sender's email server may be black-listed

If the sender's email server has been reported to Spamhaus for sending spam, then the zootzone.com servers will reject their delivery attempts.

Solution:

The sender's email provider should check whether their email server is black-listed here and if so, make arrangements to be removed from the list.

4

The sender's email server may not have a reverse DNS record

All servers offering public Internet services such as email, are required by Internet standards, to have a setting called a reverse DNS record for the IP address which the server uses.

This policy may be disabled, per mailbox, but because it's one of the primary barriers against spammers, we advise against requesting for this check to be disabled.

Solution:

The company controlling the sender's mail server must ensure that their mail servers have reverse DNS records configured for their IP addresses.

5

Your local spam filter may have quarantined the message(s)

Solution:

Ideally, you should disable your email program's spam filter, because running two separate filters usually results in one interfering with the other.

6

Your local anti-virus scanner may have quarantined the message(s)

Solution:

Check your anti-virus program's quarantine folder - there's a chance that the messages are there.