In most operating systems it's possible to specify which programs to use as the default browser for viewing web pages and the default mail program for sending emails. Other programs often rely on those settings. It's ''possible'' that your accounting program has a setting to tell it how to send emails, but I think it's more likely that when you tell it to send one it asks the operating system what program to use -- and if the operating system doesn't know the answer, the process stalls. So I would start by checking that Thunderbird is set as the default mail application -- this will be a setting in your operating system preferences.