1. Initial situation
You have an email address max.muster@musterdomain.ch . Mail from max@musterdomain.ch also arrives here . Usually emails are sent from max.muster@musterdomain.ch , but now you would like to send an email from max@musterdomain.ch .
2nd solution
- Open eM Client
 - Menu> Accounts
 - Select the affected account
 - Under General you will see the E-Mail field and next to it Aliases, click on Aliases
 - Click on Add Alias and edit the fields
 - End with OK> OK> Save & close
 - If you are now composing an email, you can select the appropriate alias address next to the send button.