Google seems to have broken email forwarding
Obviously good email (deep in a conversation) being declared "spam" has been a problem with my business for some time also.I figure Google takes all email in a 2 step process (either intentional or accidental)   1: get a good fraction of the world on GMail

   2: intermittently declare any non GMail mail spam (
      greatly lowering the utility of non GMail).

This isn't about any mass mailings or even mailing lists.  This is about directly replies from me to people I have been emailing for months all of a sudden ending up in their Google Spam folder.  Combining that with Google's good reputation and it comes off like "my dog ate my reply" (even though the mail is in their spam folder, the point is it is easier to convince a Yahoo user something is wrong on their side, than to convince GMail user something is wrong on their side).  And it isn't that I all of a sudden decide to include a lot of links, attachments or zip files.And for the "I don't like conspiracy theories crowds."  From the first this said "either intentional or accidental".  It would be enough for this to be a fortuitous bug for the effect to hurt non GMail users disproportionately.  Google could even fail a fraction of GMail and the overall image would still be "email is flakey in general, we'd better all switch to the service provider for safety."I known spam filtering is hard- but GMail has some really strong signals that I would think could dominate here (like incoming email contains text unique to a recent outgoing email).  Or email is from a sender you clearly have a relation with.(note: "Google takes all email" is a possible outcome, not a plot/conspiracy.  A bug like this on a system of this size can have a big impact.  Of course that does mean somebody running a system of this size might have an extra responsibility to look out for such things.)