I have a number of virtual servers, and they run Logwatch.  They mail the daily Logwatch reports to a local user which forwards them to my gmail account.

Sometimes, Google marks them as spam because sometimes they contain spammy URL’s, source domains, etc.  Recently however, they stopped arriving from one server, and following the rejection note, it appears Google has started blocking spam at source.

host gmail-smtp-in.l.google.com[2607:f8b0:400d:c02::1b]    
said: 550-5.7.1 [******] Our system has detected that    
this 550-5.7.1 message is likely unsolicited mail. To reduce the amount of    
spam sent 550-5.7.1 to Gmail, this message has been blocked. Please visit    
550-5.7.1 http://support.google.com/mail/bin/answer.py?hl=en&answer=188131    
for 550 5.7.1 more information. 7si10895397qeh.110 - gsmtp (in reply to end    
of DATA command)

Google isn’t blocking all the mail from my server, and it sends quite a bit to various destinations, so this is just because of the content of the message (which is a standard Logwatch formatted text e-mail).

I guess it was inevitable, and maybe they’ve been doing it for some time, but now you can never be sure that your mail is arriving at Google, and you’re not longer sure you’re seeing everything even if you check your spam folder.