[NAME, COMPANY AND ADDRESS, ex.
Jill Smith
XYZ Inc.
1234 First Street
Suite 567
Anycity, Anystate  85245]

Dear [NAME, ex. John Smith], 

Thank you kindly for the hospitality you exhibited in hosting [NAME OF SPOUSE IF APPLICABLE, ex. Sara] and me for dinner the other night.

It was a pleasure to get to know you and [NAME OF HOST'S SPOUSE, IF APPLICABLE, ex. Ron] better. 

It is great to know that I have colleagues like yourself in the company. We hope to reciprocate in the near future. 

Best regards,

[YOUR NAME, ex. John Jones]