Is it permissible to work for a competing company and send introductory offers to new companies that include clients of the former company?

If the first company stipulated that you should not work for a competing company for a certain period, and you agreed, then you are not permitted to work for another company during that period, because conditions that do not contradict the Sharia are valid.

If the company did not stipulate this, then you may work for any other company.

What an employee acquires during their work: 1. Experiences and Skills: There is no objection to benefiting from these, as they are the basis of their employment. 2. Confidential Information and Secrets: Disclosing these is forbidden due to the violation of privacy and harm to the company involved, as they are a trust and a covenant. 3. Information and Data (such as customer lists): What the company spent money to collect (such as its systems, proprietary lists, and programs) may not be copied, taken, or utilized when working for a competing company unless with the permission of the first company. What the employee obtained through their own effort, such as relationships, numbers, and mailing lists for customers and contractors, they may benefit from in their subsequent work.