If privacy is of utmost concern, we recommend that you refrain from communicating any personal data to us since plain-text email is not a safe media for communication. If necessary, use PGP-encrypted email.
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We do use a third party to operate our email service, so we remind you to carefully read #1 again.
from: https://mullvad.net/en/help/no-logging-data-policy#email
They oughta say who though.
I can agree on that, but their dns already do for anyone who is interested to find out.