How to increase the maximum allowed attachment size in Webmail

Introduction

When changing the maximum allowed attachment size in Webmail, two changes are required to be made. cPanel's internal PHP settings need to be modified, and the maximum message size needs to be increased in Exim.

Procedure

1. Log into WHM as the 'root' user.
2. Navigate to "Home / Server Configuration / Tweak Settings."
3. Click the "PHP" tab.
4. Select the "cPanel PHP max POST size (Minimum: 55; Maximum: 2,047)" textbox.
5. Enter the desired value in the textbox.
6. Select the "cPanel PHP max upload size (Minimum: 50; Maximum: 2,047)" textbox.
7. Enter the desired value in the textbox.
8. Click the "Save" button.
9. Navigate to "Home / Service Configuration / Exim Configuration Manager."
10. Click the "Advanced Editor" tab.
11. Scroll down to the "Add additional configuration setting" button.
12. Click the "Add additional configuration setting" button to add a new empty configuration option.
13. Select "message_size_limit" from the dropdown menu of the new configuration option.
14. Enter the desired value in the textbox. 
15. Scroll to the bottom of the "Advanced Editor" tab.
16. Click the "Save" button.

Please note that email attachments are encoded using base64 encoding. Base64 encoding typically results in a file attachment size around 30 percent larger than the original file.

Additional resources

Tweak Settings: PHP

Exim Configuration Manager: Advanced Editor