I have no problem sending emails using my hotmail account from my iPhone, iPad or from a hotmail sign in on the hotmail web site. The problem is only on my MacBook.
I am using Version 10.13.4 (17E202) on my MacBook.
The out going Mail Server is set at smpt-mail.outlook.com and connection is ticked to automatically manage connection servers.
I also have an iCloud account on my MacBook which functions as it should so when an email cannot be sent using my hotmail account I have been selecting this account to send the reply. Not ideal as contacts are not sure which account to use even though I receive mail from both my iCloud and hotmail accounts.