emersion/go-smtp upgrade to v0.16.0 introduced breaking changes

See https://github.com/emersion/go-smtp/issues/208

v0.16.0 changes the API of the library (see https://github.com/emersion/go-smtp/compare/v0.15.0...v0.16.0). I have downgraded to 0.15.0 for now. I may either fork the library, or do some upgrade work. Upgrading to the new API was quite easy, but the tests have to be re-written. See https://github.com/binwiederhier/ntfy/pull/568 for a WIP