Trademark infringement?

Trademarks are based on the use of words, images, etc. to identify the source of the product. Infringement occures when someone else is using a similar enough word, image, etc. to sell their products to lead consumers to believe their products are originating with you. Likelihood of confusion (infringement) analysis requires an analysis of the similarity of the goods being sold and the distinctiveness of the mark, amongh other factors. You have not identified the product you are selling or the product they are selling. The trademark DELTA, for instance, can be used by multiple companies to sell various products without creating confusion in the marketplace. MICROSOFT cannot be used the same way. You have not provided enough information to elicit a yes or no answer, but I would suggest you have disclosed enough that it would warrant a visit to a local IP attorney that offers free consultations to get their thoughts with the facts you are hesitant to disclose here.