You are describe a lot of different issues, but at least one is probably identifiable from the description. And that one is delayed receipt. Many providers (I the USA anyway) use a system of simply dropping attempts to deliver mail from servers that fail a range of tests like DKIM, SPF Reverse DNS and on advice of some reputation providers. The idea is spammer simply do not care about individual emails so they refuse to accept mail until thew sending server has attempted to deliver it more than once, or whatever threshold they have defined. Usually the ones that are delayed are from small business that often have no technical department and have not set up their email to meet all the right standards. One of those occasions where the technical kid from next door that has huge scores on some game is the company system integrator. But email like snail mail in the past comes with no guarantee of delivery at all. best efforts is all you will see in any of the providers service standards. There is no guarantee of delivery, so when you provider says there is no issue, that does not mean they are not deleting the emails as spam, it just means they are following their standards and procedures. You mention most of the problem ones are sign in type emails. These are regularly send to spam. Have you checked the spam folder in your providers web mail? They are commonly placing mail there and not even offering it for download to those using POP.