[ZWoz]

Well, for personal matters I don't write usually letters without needing feedback. When I get answer, then my previous mail is clearly arrived. My friend has forwarding to gmail address, so he can compare both ends. Over years there isn't been more issues than mentioned spamhaus incident.

[baobabKoodaa]

Ok, so you actually haven't measured your deliverability in any way. Yes, you can probably get emails delivered to your friends who have already emailed you in the past. Spam filtering is more of an issue when you try to email someone who has not engaged with you. If you actually go out and run some tests, you will probably find out a good portion of your email is blackholed by Google and Microsoft.

[ZWoz]

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.

[baobabKoodaa]

> How to you really test that?

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.