IMAP error only outside the office

[analistasis]

When you try to synchronize an IMAP account out of the office, the connection is terminated unexpectedly by the server.
The firewall ports have been checked and the error only happens with email boxes with large sizes (2GB), smaller function normally.
Tests performed with Outlook 2010, Outlook 2013, Live Mail and Thunderbird. With all the error persists.
I use MailEnable Professional Edition version 8.56.

I ask for help, thank you.