Some of my out-going email messages are being returned MAILER DAEMON?

@n7.bullet.mail.ac4.yahoo.com. The messages I am sending are correctly spelled.........would someone tell me what is wrong? Has my PC been compromised or hijacked?......I am Paula and would so much appreciate some help....ASAP....this is really affecting my livelihood negatively as I am a self-employed free-lance writer....again, plese help...am using IncrediMail and Windows Explorer..my system is well-protected.