As I said in the subject, I'm having issues with Producteev notifications getting past Google Apps' spam filter.
The easiest way to fix this for me and anyone else using Google Apps would be for you to implement an SPF record in your DNS (Sender Policy Framework - Wikipedia, the free encyclopedia).
There's also some Google documentation here: About SPF records - Google Apps Administrator Help
Please let me know if it is possible
Even though there was no reply, it looks like the SPF record was set in producteev.com's DNS zone.
That seems to have fixed my problem.
I hope it did the trick for others as well
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