Here are the best tips to determine these mistake:-
Outlook.exe Process End
Outlook to be begun in Safe mode
Incapacitate Outlook's add-ins
Close Down Unnecessary Programs
Reestablishing Outlook Data Files
Decrease letter drop size
Keep away from Interruption while Syncing
Incapacitating Anti-Virus
Fix MS Office programs
Secure and Safe Method
Read more here:- https://www.datarecovo.com/unable-to-send-emails-from-ms-outlook/
Methods to Fix Outlook Send Receive Error
Method 1: Disable Windows Firewall and Antivirus Program.
Method 2: Check for Suspicious Emails.
Method 3: Correct the Outlook Email Settings.
Method 4: Clear Outbox and Change Server Time-out.
Method 5: Run Outlook in Safe Mode.
Regards, James