Using classic Outlook for Windows in business environments
Hello John Uva,
Welcome to the Microsoft community. We have also received some feedback from other users on this issue recently, so we need to collect some relevant information in order to better help you.
Are you using the Outlook desktop application? What version of Outlook are you using?
- In Outlook, choose File.

- Select Office Account. If you don't see Office Account, option, choose Help. You'll see the product version on the right side of the page.
- You'll find your version and build number under Product Information. Please share with us the complete screenshot of the product information interface.
Did you update your Outlook application before this problem occurred?
Are you having the same problem using Outlook app in other devices? Or do other people in the same group as you experience the same situation as you?
Did you get any error message when you couldn't send email? If so, please share the error message with us so that we can better help you.
What mailbox have you configured in Outlook? Gmail, Outlook, or another type of mailbox?
What protocol did you configure the mailbox with? POP, IMAP, Exchange?
Can you send group emails via webmail? Does it also happen that the mail is undeliverable and sent back?
Looking forward to your reply again. We have to go through a series of steps to find the real cause of the problem. If you still have problems, please let me know and I will continue to help you with the problem.
Sincere greetings
Mavis - | Microsoft MSFT Community Support Specialist