To set @swisscows.ch as default sending address, open swisscows.email and do the following:
1. Open your @swisscows.email account and click on "Settings".
2. Under "Identities" click on the @swisscows.ch email address.
3. Now under "Settings" activate the slider "Set default".
4. Click "Save" to apply the desired settings.
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And just like that, you can also change the settings back to @swisscows.email as the default sending address.