SMTP, why is it that only certain emails are "sent but not delivered to the other party"?

Mediawiki is running in CentOS.

Browse to Mediawiki (http://wiki.example.com),
This system allows regular users to write to wiki once they register their email address.

System: [email protected]

User 1: [email protected]
User 2: [email protected]
(Previous to @ is dummy, domain name is real)

Mediawiki's system ([email protected]) automatically sends e-mails to users during account creation, password reset, and so on.

When you create an account, you will receive an email with the subject line Confirm email address.
user1 and user2, both of which have received email address verification.

When the password is reset, an e-mail with the subject line account details is sent.
I tried user1 and user2, but neither of them received the account details.

https://outlook.live.com/
Logged in to the above site with [email protected] and checked the submitted box.
The account details email has already been sent.
There are two copies addressed to user1 and user2.

Why can't I get an email when I've already sent it?

If you haven't received any mail, it's likely that Mediawiki's SMTP settings are incorrect.
However, since the mail address verification has been received by the user, it does not appear to be an SMTP misconfiguration.

I thought it was distributed to spam, but there is no spam in the spam folder.

user1 uses Yahoo Mail and user2 uses a mail service called disposal email.

Checking email addresses is not classified as spam, but
Account details may be classified as spam, but
Is it unnatural that Yahoo Mail and Discarding Email behave exactly the same way?

Would it be possible if different mail services used the same filtering engine?

Hasn't it arrived in the first place?
Has it been delivered to the receiving server, but it was thrown away before it was delivered to the user?
I want to separate , but I don't know what to do.

Alternatively, SMTP (outlook.live.com) has some sort of filter on it and
Is there a possibility that you didn't actually send it by pretending to have sent it?

Summary

If you have already sent an email but you do not receive an email,

·How do I check if it's really being sent?
(If there is an email left in the sent box, can I make sure it is sent?)

·If it is classified as spam, how do I detect that it is classified as spam?
(Yahoo Mail and Discarding Mail services may not even show up in the spam folder, but may implicitly erase the mail?)
If there is a free e-mail service that doesn't automatically categorize it as junk e-mail and can receive e-mails without a guard, please let me know.
I would like to separate whether it has not arrived in the first place or if it has been filtered and cannot be seen by users.

"·If you have any other advice regarding this phenomenon where ""email when creating an account"" is received but ""email when resetting a password"" is not received, please let me know."