I want to ask a question, asking for recommendation and pro/con about which MTA (e.g., exim & postfix) on what OS (Linux/FreeBSD). Will that be acceptable?
Yes, you can ask for software recommendations here (whether amongst a fixed set or not). Just make sure your question states your requirements precisely. “What's the best MTA?” is not a good question (best for what?). “What's a good MTA and OS combination for a server dedicated to relaying and archiving 10k mailing list messages per day?” is a good question.