Phplist doesn't send out email to outlook, but send out to Gmail

Hi guys,
For some reason our PHPLIST stopping sending email to outlook and few other email services.
For example: when I send it out a testing email to, it said “Sent test mail to: Success” . But I never received the email, I checked my spam and junk folder, but it’s nothing there.
When I tested with Gmail, I received the email. That’s so werid. Do you guys have any clues?
please let me know. thanks.

@Benc Most likely this is because your mail server has been blocked by Outlook. SMTP success messages from remove mail hosts are not always truthful. Sometimes mail is discarded but reported by Gmail and Microsoft as delivered. Either way this is an issue you must persue with your MX / IP configuration. You can check the reputation of your IPs generally, and with Microsoft specifically. There are many things you can do to improve your reputation with MS which are generally outside of the scope of this forum.