Sending to @e.email: "Server certificate not trusted"

After trying to send an email from my Proton Mail address to my e.email address I got this response from Proton Mail:

This is the mail system at Proton Mail.

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THIS IS A WARNING ONLY. YOU DO NOT NEED TO RESEND YOUR MESSAGE.

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Your email could not be delivered for more than 12 hour(s).
See below for the delivery error.

It will be retried until it is 2 day(s) old.
If still not successfully delivered by then, our system will stop retrying and send you a bounce email.

The most common reason for delayed email is unreachable recipient mail server.
Learn more about delivery errors in our knowledge base: SMTP errors | Proton

<x.y@e.email>: Server certificate not trusted


message/delivery-status

Reporting-MTA: dns; mail-41103.protonmail.ch
X-Postfix-Queue-ID: 4WGl9q5q62z4x2dh
X-Postfix-Sender: rfc822; x.y@pm.me
Arrival-Date: Sat, 6 Jul 2024 22:17:19 +0000 (UTC)

Final-Recipient: rfc822; x.y@e.email
Original-Recipient: rfc822;x.y@e.email
Action: delayed
Status: 4.7.5
Diagnostic-Code: X-Postfix; Server certificate not trusted
Will-Retry-Until: Mon, 8 Jul 2024 22:17:19 +0000 (UTC)

Is this a known issue?

In the meanwhile the email was delivered.