We have been made aware of a crash when opening the in-game mail system. This issue is caused by AVG Anti-Virus software preventing Stronghold Kingdoms from opening a new 'window' within the game.

A resolve the issue add an exemption to AVG for the folder

C:\ProgramData\Firefly Studios\Stronghold Kingdoms\

Please follow these steps or click on the link below:

1. Open your AVG program.
2. From the Options menu select Advanced settings.
3. In the left pane select Exceptions.
4. In the right pane click Add exception.
5. In the drop-down menu please select the exception type.
6. Browse to the file, folder that you want to exclude or type URL 
7. Click OK to save details of the exception.
8. Click OK again to save the settings.

https://support.avg.com/SupportArticleView?l=en_US&urlName=How-to-exclude-file-folder-or-website-from-AVG-scanning