Might be your recipients ISP.

my ISP uses a very aggressive spam filter on my POP3 email service and to their credit, I rarely receive any spam, even on an email address I've had for over 10 years. However, their filter does have an element of false positives, about 1 or 2%, I think. None of the spam messages are sent to my email client (Thunderbird) so never show up in the junk or spam folders. Instead, the ISP holds them on the server for 10 days, then deletes them automatically. The only way to see these messages is to logon via webmail, whereupon, the server spam folder is visible.

If you know a message was sent but hasn't arrived, it may be worth your recipient checking their ISP server folders, where this is possible.