1. Check Outlook settings for allowed domain

• Open Outlook Settings > Accounts > Your Account > Advanced Settings
• Check the allowed domain if it is not set properly then change it as required.2. Update Outlook and related plugins

• Open Outlook and check for updates
• Download and install the latest version of Outlook or its related plugins if available.3. Check the sender’s email address for any typo

• Contact the sender to ensure that their email address is correct.4. Check the spam folder

• Check for the messages from the sender in your spam or junk folder.5. Try connecting to alternative email service

• You may try using a different email service (e.g. Gmail) and try sending an email to the sender to check if it is received.