I remember a saying from somewhere. It was similar to:
"Have a new idea for an application? Great. Join one of three projects already implementing it."
The idea was to promote having a few strong programs in the Hurd for a task instead of many weak ones.
I recommend you instead look into Thunderbird
If you are set on rolling your own, identify what operating system(s) it will run on, whether it will be terminal or GUI based (and then which dependencies), and which features you want included.