I know that, there are two server in a mail server these are SMTP and POP3 server. POP3 (Post offict Protocol 3) server stores mail. But how is do it and is there any memory limit? Or POP3 server can reject any mail?
Yes, there is a maximum memory limit of POP3 mail server. The maximum limit of connection (authenticated or not authenticated) that POP3 can achieve is 25,000. The default value of POP3 is 2,000. It can be changed between 1-25,000 depending on what value you input to its settings. I will also teach you how to change the POP3 memory limit.
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Just follow these steps:
1. Click on the system services icon.
2. Click on the mail server button.
3. Click on the MDA settings button.
4. Find the MDA Server Information (POP3).
5. In the text box of POP3 Memory Limit Field, type the amount of memory you desire POP3 mail server to accept.
6. Click the update button.
7. After clicking the update button, this will appear on the top of your screen. 
POP3 is Pop Office Protocol is a standard protocol used to local emails allowing you to download messages from server of to your computer.
This allows you to store data, email messages unto your computer hard drive off from webmail server. However, POP3 service is dependent on the email provider's email capacity to download large files for users. Limiting POP3 to the allowable data files provided by your email or Internet Service Provider.
Most email provider has limitation on how large files you are allowed to download from the webserver. Email provider allows webmail storage limiting from 100MB up to 10GB depending on the service they offer.
In most cases, a free email address, such as Gmail, Yahoo mail and other email provider gives up to 7000 megabytes of data or more. This limits you only up to this data download for POP3 data files.
