All email marked as spam

Description

All e-mail sent to new users, asking them to activate is marked as spam. On some mail providers, it is placed in the Spam-folder. On others, it may be rejected with the following error in Mobilizon log: 554 5.7.1 Spam message rejected

I contacted my mail provider (https://migadu.com), and here's a couple of things they noted about the e-mail:

There are multiple issues with the headers and content. Most importantly, you must have message-Id and date headers.

Next, please check if you do not use short link images.

Furthermore, it seems there is heavily Base64 mixing inside of the mail and its subject. Please check that you use consistent content type. Maybe have an email program assemble the message for you (e.g in Drafts) then view source and copy.

The message itself you are sending matches known spam so you might want to re-adjust the text a bit, as also receiving ends might have the same input as we do.

I also made this analysis (Score and Description) using this site https://www.ipqualityscore.com/email-deliverability/email-spam-test-checker/:

0.0 No description available.
0.0 BODY: HTML included in message
0.0 From: base64 encoded unnecessarily
1.2 Relay reports its domain incorrectly
1.4 Missing Date: header
0.1 Missing Message-Id: header
1.3 Delivered to internal network by a host with no rDNS

Steps to Reproduce

Configure mail gateway
Register new user
Activation-mail is marked as spam.

Expected behaviour:

The email should get delivered without a problem

Actual behaviour:

All email is marked as spam.

Reproduces how often:

100% I guess.

Versions

Docker image framasoft/mobilizon:master from 10. Nov. 2020

Additional Information

My e-mail gateway is configured with DNS, DMARK, SPF and so on.