Thanks for the question.
In 21.co you need to register as a sender to send a message and the address 21.co/myname is not as familiar. Making a resitration with passwords etc is very unwelcoming to users.
Also with our service you just use normal email, you get it delivered on your gmail,POP and whatever other clients you are using. There is no need for a website. 21.co wants to build a whole social network and support other actions that will get you money etc.
Veropost is just an email front.
We also support more donation organizations and will make the list bigger.
Fun fact: I started the project before them and actually explained the whole idea to Stripe in Sept. in confidential terms. We learn from mistakes