I received an e-mail that had an attachment. The file was a .eml file, but either I couldn't open it, or it opened in Notepad, or some other application, or MailCOPA tried to compose new mail with it as an attachment.

Go to Configure Set Preferences General and check both Check if MailCOPA is default mailer and Always set MailCOPA as default email program

MailCOPA Email Program General Preferences

 

Close down the MailCOPA email program.

Open the windows control panel, by clicking on the windows "Start" menu (at the bottom left of your screen, usually) selecting Control Panel, or Settings and then Control Panel depending on which version of windows you are using.

If you are running your control panel in "Classic View" select "Internet Options".  Otherwise select "Network and Internet Connections" and then "Internet Options".

Click on the "Programs Tab" and use the "E-Mail" dropdown to select something other than MailCOPA.

Press OK to close the Internet Properties window.

Restart MailCOPA and your .eml attachment should now open correctly.