Symptom: emails arrive unpredictably if at all
Cause: We use a service (Mimecast) which aggressively rejects spam using the technique greylisting. Mimecast soft bounces messages and waits for the delivery service to retry.
Because Amazon SES always uses a unique from address, greylisting is always applied.
In theory this is fine - Amazon SES copes with soft bounces and will retry - https://forums.aws.amazon.com/thread.jspa?threadID=76733
However Amazon SES does not seem to retry in a predictable way, or maybe retries with new unique from addresses which would attract greylisting again.
Workaround: we configured our mail provision to skip greylisting for all Amazon SES messages (we could not specify Bubble). This is unsatisfactory because we may now get spam if it is generated through Amazon SES.
Not really sure this is a bug - probably just an inevitable consequence of using Amazon SES; it certainly is in some ways the problem of the receiver. However we want in general to be sure emails are received when sent, and I wanted to share the information for others who might find it useful.