In the professional world, it is very important that you know the tricks of the trade. You should be well equipped with the manners especially if your business deals with personal relationships.
Writing a business welcome letter shows the other party that you value this relationship. It is an effective tool to introduce your company to new partners. It makes a new-comer realise that he/she took the right step to build this bond. Moreover, it is a decent and courteous way to develop your business. You can build, or at least give an impression, a personal relationship with your new customers, employee or business partner.
In the opening paragraph, you should start with a couple of chivalrous sentences showing that you are really happy to have the other party in a business relationship.
The second paragraph will comprise brief but complete details of your contract/ deal and a hope that this will turn out as the right decision for both companies/ parties.
Next paragraph will give your addressee impression that you trust their abilities and will look forward for some valuable feedback.
Close your letter in a courteous manner and greetings followed by your name.