Step 1: Verify Internet connectivity
Step 2: Check for cause of problem in the bounceback message
Step 3: Verify account settings (POP / IMAP)
Step 4: Change outgoing port to 5125
Step 5: Check for an Outlook error code and refer to our common error code resolutions or visit the Microsoft knowledge base
Step 6: Click here if a message is stuck in the outbox
Step 7: If sending large attachments, increase the server time out
Step 8: Disable email scanning