Add List-Unsubscribe header

[githubrsys]

For better spam scoring it is helpful to have a List-Unsubscribe header.

See solution here:
http://stackoverflow.com/questions/4365850/list-unsubscribe-in-e-mail-header-how-to

Unfortunately it only works perfect with a mailto: URL . So there would be a bit more work neccessary

[Adrien Crivelli]

Great idea. Planning to automate that in next release if a bounce account is available.