This is a bug about the bugtracker itself (and/or the http://eigen.tuxfamily.org domain configuration).
Gmail classifies emails from the Eigen bugtracker (this one) as likely spam. This is very likely because of a faulty or missing SPF configuration. Basically a record needs to be added to the domain's DNS record so that clients receiving the email can verify if the server that the email is coming from is allowed to send that email from the domain.
See the two screenshots for how it looks e.g. in Gmail, but many (most?) email providers will give emails that do not pass the SPF a very high spam-likelyhood.
See e.g. this for more info on SPF: http://www.openspf.org/SPF_Record_Syntax
The same has happened in the past with emails from the Eigen mailing list too actually, it hasn't happened recently I think (because I told Gmail to explicitly classify them as not-spam), but it might be worth checking.
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