Answer

PROBLEM

Some POP3 mail server providers do not accept five or more simultaneous connections which is the default setting for the GFI MailEssentials Pop2Exchange module. 

In this cases the GFI MailEssential Pop2Exchange logs would indicate something similar to the below:

ENVIRONMENT

• GFI MailEssentials 2012
• GFI MailEssentails 2014

SOLUTION

1. Open Regedit.exe (Start >Run >regedit and hit Enter)
2. Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\GFI\MailEssentials\Antispam\POP2Exchange (in case of x64 bit operating systems to HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\GFI\MailEssentials\Antispam\POP2Exchange)
3. Create a new DWORD Value called Simdownloads with the decimal value data of the desired number of simultaneous connections
4. Restart the GFI MailEssentials Pop2Exchange service 

CAUSE

Some POP3 servers limited the number of simultaneous connections to a number smaller than 5.