There are many more IT professionals out on the market right now due to layoffs and dot-com implosions.
Unemployed professionals are more active than ever, often flooding recruiters with resumes.
Mainframe professionals have it tougher than most. Recruiters don't really know what they do and really don't want to know. Corporate recruiters are concerned about keeping their jobs so the more strategic hires that they make, the better they feel. Both in the body of your e-mail and in your cover letter, match your qualifications to the requirements of the position. Don't assume that the recruiter has any technical knowledge.