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)
Enigmail used to use gpg-wks-client. @kai implemented it back then and we had a milestone meeting to show that it works with posteo.
I know, I helped implementing that. Patrick changed it.
An argument could be that Enigmail is currently vulnerable to WKD stalling, Keyring pollution etc. everything that dirmngr protects against.
Enigmail's support for Thunderbird 68 ends in two days:
https://www.enigmail.net/index.php/en/home/news/71-2021-08-31-end-of-support-for-thunderbird
Enigmail is still available for Interlink and Postbox. It would possibly make sense to improve the WKD/WKS support for those client.
Given the decreased importance of Enigmail, I think such a request should be handled in the Enigmail issue tracker at
https://www.enigmail.net/index.php/en/support/reporting-defects