Sometimes Outlook might have problems sending a message. There may be various causes- corrupted PST, AV scanner etc. The message, instead of going to the Sent Items folder, sits in the Outbox indefinitely.
To remove it you can use a free tool called MFCMAPI
This video shows how to quickly and easily add an email signature for all users on Exchange 2016. The resulting signature is applied on a server level by Exchange Online.
The email signature template has been downloaded from: