If I answer an incoming mail in Outlook, the action doesn’t take place via my own e-mail address, but via the address of my second account or alias. How can I prevent this?
Both Outlook and Outlook Express use the account to which the e-mail was sent. So if you click "Reply" or "Forward", the address is normally the same as the one to which the incoming mail was sent.
However, if the accounts are aliases for the same mailbox on the server, one of the other addresses might appear instead of the normal address (alias) to which the message was sent. It depends on the (alias) account that retrieved the messages.
The quickest way to choose the right delivery address in Outlook, is to press the button "Accounts" and to select the right account. A rule will appear at the top of your message: "This message will be sent via email@example.com" (The account/mail address of your choice).
If you are used to use the same account to send (and answer) e-mails, you can make it your default e-mail address. Go to Extra -> E-mail accounts. Select "View or change existing e-mail accounts" and press the button "Next" (in Outlook 2016: File -> Info -> Account Settings).
Select the desired account and choose "Set as default". Click "Finish" to confirm.