I think the vastly different experiences have a lot to do with the quality and scale of the ISP. Best results with small, good quality ISPs. Also running my ow

I think the vastly different experiences have a lot to do with the quality and scale of the ISP. Best results with small, good quality ISPs. Also running my own servers for personal and business, and working well. But when we tried to use one of the large VM providers the experience was much less reliable. Despite ensuring the IP was not on the various block lists etc. mails would be accepted and silently discarded by recipients ISPs, perhaps due to the level of abuse of these IP ranges.