[dspillett]

Still running my own mail server, for myself and friends. No problems with deliverability thus far (including to Google) with SPF and DKIM set correctly.

The killer problem is often source address, and source address range, reputation. Mail sent from a normal residential ISP will have trouble. Addresses allocated by many VPS providers (and some dedicated server providers) will have spotty reputation. Mail from cheap VPS providers simply won't get through (many block SMTP outgoing and state so on sign-up because they don't want the support burden of people complaining about this). Sending mail from cloud providers can be an issue too (I've had mixed results with VMs in Azure sending out alerts).

If you are stuck with such a source reputation issue then you can use a service like MXRoute (several others are available, this one I've not used but it seems popular on a couple of hosting forums I frequent) as a relay. Obviously that is not free, but it is cheaper than moving to a much more expensive VPS provider or changing your home ISP.

Another reputation issue many have when setting up a new project is that many mail servers explicitly distrust new domains. In this case SPF, DKIM, and a good reputation source address won't make much difference. The definition of “new” varies from place to place but where it is an issue you need a domain registered at least some months ago. This can be mitigated somewhat by users explicitly marking messages from new domains as not spam, but that is not something you can rely upon as said users need to see the message to be able to mark it that way.