mail not sending

I have an iCloud acct and this morning was talking on the phone to someone I sent an email using the icloud acct out going server. They said they never received it. While still on the phone with them I resent the same email and again using the icloud acct out going server. They said they still didn't receive it. So, at that point I resent it using an email account I've had for years using their out going server and it wasn't 5 seconds and they said they got it.

All the emails were sent using Apple Mail on a MacBook Pro running Monterey and all the emails showed up in the sent folders, yet the person i sent they didn't receive the one's sent from the icloud acct out going server.. I checked and both accounts say their online, I just wonder where are those emails going if they aren't going to the person I set them to and it's got me not trusting the icloud server to send emails.? Anybody have any idea what may be the problem?