I also realised I hadn't made an spf record for our domain and some email notifications to Gmail had failed on occasions (due to number of subscribers on Gmail). Should be sorted now.
(Triggered when sending similar emails to many Gmail accounts at once and Gmail thinking it was spam). If we still get issues, I can look into other authentication methods they recommend.
awesome! maybe already on your list, but one thing that could do with tweaking is the email notifications - currently they're busy HTML messages with a huge banner image, rather than the previous text updates. Personally I find the new ones a bit harder to parse.