Posteo only accepts keys without a username for WKD / WKS publishing.
Enigmail, by default, does not create such keys. Either the key creation in Enigmail needs to be changed or GnuPG's wks-client should be called and filter / create a new userid on the fly.
Currently Enigmail does not use GnuPG's WKS tools so that older GnuPG versions are also supported.
(btw. same goes for gpgol)