Email delivery problems usually fall into a few categories. Work through these checks first.
Check the basics
Confirm you're connected to the internet and signed in. Check your Junk or Spam folder for missing messages.
Sending fails
If you can't send, note the exact error. A full mailbox, a blocked attachment, or an outbound filter can all cause this.
When to contact BTS
If multiple users are affected or email is down entirely, submit a support request with urgency 'Critical' and category 'Email or Microsoft 365'.