The default option in SliQ Invoicing for sending invoices, quotes etc. by email is that SliQ will use your default email program, e.g. Microsoft Outlook or Live Mail.
When you send an email via SliQ, SliQ doesn't directly launch your email program but instead asks Windows to launch whichever email program you use on your PC. Sometimes Windows can fail to launch the program if it isn't sure which email program is the default one to use.
Correcting the error is almost always as simple as telling Windows which is your default email program.
For help on setting the default email program in Windows XP, Vista or Windows 7, see troubleshooting email