You may see an entry like the following in the /var/log/exim_rejectlog following a failed delivery.
2020-09-16 04:50:27 1kIOSg-0000uY-Ln rejected from <firstname.lastname@example.org> H=(localhost.localdomain) [127.0.0.1]:48388: message too big: read=60877083 max=52428800
This is caused by the email being larger than Exim's message_size_limit setting. Emails larger than this setting will not be sent.
The message_size_limit can be increased in WHM following the steps below. Note that any attached files will be Base64 encoded, which will increase the size each file uses in the email, so you will need to increase the limit to higher than the increased size of the attachments. Base64 encoding will increase the file size by about 40% as described here.
1) Login to WHM and navigate to the following location.
Home » Service Configuration » Exim Configuration Manager » Advanced Editor
2) Set "message_size_limit".
- If the "message_size_limit" setting is already present, increase it.
- If the "message_size_limit" setting is not present, click the "Add additional configuration setting" button at the bottom of "Section: CONFIG" and add an entry for "message_size_limit". The default for this setting is "50M" so it should be set higher than 50M if the setting was not already present.
3) Click the "Save" button at the bottom of the page.