Go to Parties" - "Relations".
Click on the tab for "Business relations".
Approved relations with business parties, and pending relation requests, will be listed. 
As a supplier, to be able to send messages to a party, an agreement must have been signed for that party with Btwentyfour. Even though a buyer and a supplier has agreed on a business relation in the platform, by accepting a relation request from the other party, a supplier still needs to sign an agreement with Btwentyfour to have the message relations approved for that specific buyer.
FOR MORE INFORMATION: How to see "Approved message relations" for a business relation?