Issue:  None of users are able to receive any external emails while able to send & receive internally and send externally – (Office 365) and sender might receive following Undeliverable email stating "recipient email id" wasn't found at ", or the mailbox is unavailable." 

Steps to come out of this Issue –

1- Most probably it happens when there is incorrect MX Record is added/modified, hence in order to check it out whether your MX Record in on place then access following link and put your domain name and check if it’s visible there.

2- If there is no MX found for that domain then you can double check the same from Office 365 Admin Portal > Setup > Domains > Click on your domain name > Check DNS > here you can find the list of incorrect/missing MX records.

3- If you find any incorrect records, then you need login to your Global DNS Portal (cPanel) and remove invalid/incorrect MX entry while there should be only one MX Record which looks like –

4- Once updated MX Record on your Global DNS Portal (cPanel), again follow steps (2) and make sure that there is no error and now status should look like correct.

5- Once it’s corrected now you must be able to receive email from external users as expected.

Note - Typically it takes just a few minutes for DNS changes to take effect. However, it can occasionally take longer for a change you've made to update across the Internet's DNS system.