The Comunity Forum

At Arivisti, Technical Analysts will get asked this question many times. Unfortunately the question

is a little vague since there are really hundreds of reasons why an email may not get delivered.

These can be:

Authentication issues (the most common).

DNS related issues (is your rDNS set up correctly?).

SPF, DKIM and/or DMARC settings (misconfigured or missing completely).

Firewall issues (or proxying of port 25 by the NOC, Datacenter, Host or customers ISP) or other networking issues.

Blocked IP addresses (Customers IP address may be blacklisted).

cpHulkd brute force lockouts.

SpamAssassin settings too strict.

Spammer (or more likely a compromised email account) has filled up your exim mail queue with spam messages and legit messages are now waiting to be sent.

Exim misconfiguration.

Sender Verify and even Greylisting or other temporary soft-errors.

An incorrect setting within your /etc/resolv.conf file.

A misconfigured filter.

Mailbox is over quota (5.2.2. error).

Virus was attached to message and virus scanner redirected your email to /dev/null.