How to you really test that? Any provider is autonomous, you really don't verify delivery. You have few technical options, like DSN or MDNs, but servers or clients don't need honor them.
I didn't mentioned clients, I had few of them too. Some used internal mailing list style things, group addresses forwarded some outside service, like hotmail or gmail. I didn't had any complaints from them. Now talking about personal experience, your assumption is wrong. I don't change mail only with my family or friends. When you write merchant or government agency there is even more initiative to notice missing answer.
You can test by sending email to a bunch of different mailboxes on different email providers, and then seeing which emails land. I've previously used GlockApps for this test and I've been happy with it, but you can probably find other similar services too. I'd be interested to know the results if you decide to test your deliverability.
