Your Cpanel server allows you to change the Maximum message size limit which affects attachments in emails. Here is how you can manage this limit via your control panel.
1. First log into your Cpanel control panel.
2. Click on Service Configuration
3. Now click on Exim Configuration Manager.
4. On this page click Advanced Editor.
5. Scroll down and Click Add additional configuration settings
6. Type in Message, it will bring up a few options. Click on: message_size_limit
Type in a limit of your choosing (default for Cpanel is 50MB)
7. Scroll to the bottom and click Save
Note: Your server with Nimbus is capable of being used as a basic mail server, but the ongoing battle between spam management, hacking and blacklisting means that Office 365 and Google Apps are the go-to platforms if email is critical to your business/site. Read more here.