Your Plesk 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 Plesk control panel.
2. Click on Tools & Settings.
3. Now click on Mail Server Settings.
4. On this page you can set the Maximum message size. Please note this is in kilobytes with the default of 20 megabytes (20000 kilobytes).
5. Once set click OK to 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, Google Suite and Zoho are the go-to platforms if email is critical to your business/site. Read more here.