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.