SpamAssassin is a very good spam filtering service rolled into cPanel and WHM’s email services. It’s very configurable, both as a user and as an Administrator, and is highly effective at blocking spam. However, when you attempt to filter larger messages you will sometimes see an error like this in your exim_mainlog:

**** debug string too long - truncated ****

That error generally results in a bounce-back email delivered to the sender of the message, and generally causing confusion all around. The error itself is related to the size of the message as related to the maximum size that Exim will scan through. You can see exactly what I mean in the Exim filter files documentation in the section with the following:

$message_body: The initial portion of the body of the message. By default, up to 500 characters are read into this variable, but the system administrator can configure this to some other value. Newlines in the body are converted into single spaces.

To correct this, you need only increase the size of message that Exim will scan by increasing the $message_body_visible variable. For cPanel/WHM users, this should be done through WHM’s Advanced Exim Configuration Editor.

Edit Exim Configuration

Edit Exim Configuration

Remember: an increase in the size of a message that Exim will scan also increases the amount of resources that Exim could potentially use, so only increase this value as far as is absolutely necessary.

Once you save your configuration, WHM will put the new configuration in place and restart WHM. You can then test to ensure that the new configuration will have the desired effect, and see that your clients can now receive the larger messages again!