This is an big red (ugly) warning on every e-mail notification that is send by your forum.
English translation of the warning:
"This message could be a scam."
https://support.mozilla.org/en-US/kb/th ... -detection
Potential cause:
Example:Links where the text doesn't match the server name (for example, the text of the message might say "https://secure.example.com" but the link actually goes to "http://phishing.example.com" instead). Phishers do this to fool you into going to their site. Unfortunately some legitimate mailing lists also do this with redirectors for tracking purposes.
But the link really points to:If you want to view the newest post made since your last visit, click the following link: viewtopic.php?f=93&t=17548&p=152404&e=152404
(Data truncated due to unkown data)
A possible solution
I personally don't like being tracked, but I know that this might be necessary for you.
However you could most likely fix the issue if you use some DNS features to hide that you actually call a strange URL.
(I'm not familliar with DNS setup, so I cannot explain how to setup it, but I know it is possible)
You use a domain called tracking.factorio.com and bind its IP to u233XXX.ct.sendgrid.net and
then you can send the emails with links that link to https://tracking.factorio.com/wf/click?upn=... which is far less suspicious.
I don't know whether this is enough to avoid thunderbirds warning though.
Another solution would be that host the tracking on your own page or first link to your own page that redirects to the tracking page.
forums.factorio.com/tracking/....