Here's the issue...
I have all my previous email accounts setup from a previous configuration and all of them can access their appropriate emails for receiving and sending without issue.
However, if I create a new user in Virtualmin and then try to connect this user by adding a new account to my client (Outlook in this case), it can connect to receive emails (POP3) but NOT for sending (SMTP).
I have set the account details exactly as all the other accounts which work and have also tried using port 25, 465 and 587 using the appropriate encryption types but I can never connect using the client to SMTP but CAN connect to POP3.
I can, however, connect using said account via Usermin so I'm wondering, is there something I should look for in the POSTFIX setup settings?