Is it illegal to contact prior clients from past employment to follow to new business?

My new business is conducting a large marketing mail out through a local printer. The printer has sent postcards to former clients of mine that I had when I worked for someone else, an industry related mailing list was used by them. What if one of my former clients decides to call me and change firms? How do I advise them? Do I take the client on? Do I politely say it would be a conflict of interest to have them switch firms? Would there be a legal problem as a result of taking them on? For years I've had my hair and nail techs call me and tell me they've switched to a new salon and there were no problems as a result. My business is a services firm. Thank you