Occasionally, messages go to spam. Sometimes quite inexplicably. Just the other day, an order from a customer came in and I did not see it until two days later when I happened to check the spam folder. Made no sense why it went to spam; it used similar wording to previous eMails from them that did not go to spam.
This has happened a few times, so I try to make a habit of checking the spam folder, but I don't do it every day because most days, only actual spam is in there. Probably 95% of the time, only spam goes in there.
So, it happens.
Even addresses I have whitelisted still occasionally end up flagged as spam.
Oy. And, of course, if you disabled the spam filters, you'd be flooded with junk. I have one eMail account where I don't have spam automatically sent to the spam folder. I manually send the spam into the spam folder. And I'd say maybe 80-90% of the messages I receive at that address are spam.
And, occasionally, real messages end up lost in the spam and I end up accidentally throwing them into the spam folder!
So, whatever method you use, you're gonna lose and real messages end up in the spam folder.
I hate spam.
The bottom line is that, before giving up, it's a good idea to send a follow-up eMail just in case the first one went to the spam folder because you never know.