How to change the sending mail adress?

I have installed 2 mailadresses for me. When I want to send a mail it works only with one adress. Beside this mailadress there ist a small triangle just as to chose the sending adress, but there ist only this one adress available. ???

Hello Anton,
if you have more than one mail address you should be able to choose between them in the new message window like so (just click the address to open the dropdown menu):

If no other choices are present you probably have an incorrectly setup outgoing server.
Are there any error messages in Menu>Tools>Operations? If so, please copy both the Error and Logs tab.
Check if your outgoing server is set in Menu>Tools>Accounts.