The main complaint with the software, which Microsoft released in January, is that it inserts a long jumble of letters and numbers into the subject line of e-mail sent through the system. The string of characters, which Microsoft calls a generated unique identifier (GUID), could confuse e-mail recipients, or worse yet, cause e-mail sent to customers and prospective clients to be blocked by spam filters, according to Microsoft customers and resellers.

Microsoft has even promised to refund one unhappy customer, Promarketing Gear in Bellevue, Wash., the $7,000 the company spent on the software. Microsoft touts the software, called Microsoft CRM, as a tool to help small companies save money and boost customer loyalty by streamlining sales, marketing and customer service activities.