If you are in the UK/EU and tempted by Thunderbird Pro, it seems that:
* the contract does not include a data processing agreement. It is unclear how any professional user in the UK/EU might use the service in a manner consistent with the UK or EU GDPR.
* the servers are in Germany but the agreement (and presumably MZLA) are subject to US law:
> this Agreement between you and MZLA and any access to or use of the Website or the Services are governed by the federal laws of the United States of America and the laws of the State of California, without regard to conflict of law provisions. You and MZLA agree to submit to the exclusive jurisdiction and venue of the courts located in the City and County of San Francisco, California.