The German Federal Constitutional Court has decided today that German email providers must be able to log IP addresses.

Link to their English-language press release: https://www.bundesverfassungsgericht.de/SharedDocs/Pressemit...

are they allowed to serve customers through onion routing?

Why wouldn't they be? This only says that, when approached by law enforcement with a valid warrant, they need to be able to log the IP that accessed a certain account. If that IP is a TOR exit node, so be it.