You need to check the >SMTP< server logs (that isn’t Mautic nor the web server).
What are those logs saying? (SMTP logs)
You should find traces of the email sent from your Mautic app. And most importantly, what the SMTP server did with it.
If Mautic report the email to be sent, probabilities are that the email was sent. But blocked later in the chain.
This is the chain:
PHP/Mautic (it can write but not send an email, like your Outlook) >> SMTP server (this is what can send emails) >> MX server (this is what can receive emails) >> IMAP/POP3 server (this is what allow you retrieve emails) >> Email client like Outlook/Thunderbird/Roundcube (this is what allows you to read emails - and also start a new chain, like the PHP part).
Now, since you report no problem originating from the PHP side (Mautic), you need to check the next link in the chain: STMP server (this is what can send emails)
If it does come out clean (you’ll see it with the email traces found in the SMTP server log), you need to check at the following link in the chain: MX server (this is what can receive emails)
If it come out clean, the IMAP/POP3 server
If that comes out clean, it’s your email client that is broken.