Sending mail to travel@torproject.org does not forward to gmail

I wrote an email to travel@torproject.org, which worked a few days ago, but now eugeni informed me it couldn't deliver to gmail:

This is the mail system at host eugeni.torproject.org.

I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can delete your own text from the attached returned message.

              The mail system

<@gmail.com>: host gmail-smtp-in.l.google.com[2a00:1450:400c:c0c::1a] said: 550-5.7.26 This mail has been blocked because the sender is unauthenticated. 550-5.7.26 Gmail requires all senders to authenticate with either SPF or DKIM. 550-5.7.26 550-5.7.26 Authentication results: 550-5.7.26 DKIM = did not pass 550-5.7.26 SPF [] with ip: [ 550-5.7.26 8] = did not pass 550-5.7.26 550-5.7.26 For instructions on setting up authentication, go to 550 5.7.26 https://support.google.com/mail/answer/81126#authentication g7-20020a05600c4c8700b004107664bbe8si4522016wmp.61 - gsmtp (in reply to end of DATA command) ...

At the same time, there was a CC directly to gmail from my own mailserver, which gmail accepted:

postfix/smtp[3961572]: F067B38802FE: to=<@gmail.com>, relay=gmail-smtp-in.l.google.com[]:25, delay=0.97, delays=0.13/0.01/0.41/0.43, dsn=2.0.0, status=sent (250 2.0.0 OK 1707803587 c8-20020adffb48000000b0033add90c729si3955987wrs.862 - gsmtp)