Where can I adjust the 'Send from' account for the mailing from the portal?

This can be adjusted in the admin by going to 'Settings' 'Shop' 'Emails'.

Please know that you can only use a custom domain when your IT department has added the records for this during the implementation phase. If this becomes a topic later on, we can always supply you with the right information for this but please contact support@getmarvia.com first.