With PGP Messaging enabled, you are unable to send or receive email messages.
This issue occurs if .OST or .PST mail file size is more than 4GB.
Note: Outlook 2007 limitation of .ost is 4GB. After the .ost size increases to 2GB, you may find this issue.
Check the file size of the Outlook mail file, to determine if this is the cause of the issue. The file name by default is the user’s email address - EmailAccountName.ost.
File Location - C:\Users\%User Name%\AppData\Local\Microsoft\Outlook
Reducing the size of the mail .ost\.pst file, resolves the issue.
Try using the following methods to resolve the issue:
Method 1: To reduce the OST or PST file size, permanently delete any unwanted email and other items from Outlook inbox, Deleted Items folder, and other Outlook folders.
Method 2: Creating a new PST file or Archive for old email and then re-create the PST or OST file (If the size is more than 4 GB, its recommended to move the emails to a local .pst file).
Method 3: Split the large OST/PST file into smaller files to reduce the file size.
Note: You can also use the 2 GB Truncation Utility to create a truncated copy of the .pst file that contains a user defined amount of data from the file.
You can easily download it from Microsoft download center: http://www.microsoft.com/en-us/download/details.aspx?id=12972
Microsoft Outlook 2007/2010
Encryption Desktop 10.3.1