Comment (by Roland Haas):

The mailing list emails are HTML (only) emails, which so far seem to have been ok with the usual email clients (which would prefer the HTML part even if there was a plaintext alternative). Which email client are you using?

The archive was supposed to render HTML parts into plain text as far as I can tell from the mailing list settings. I will try and see if I can find out what is going wrong or (more likely) I may be misunderstanding the mailman options.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2523/the-trac-mailing-list-could-have-more