iPhone 4S - Mulitiple mail accounts. Send mail on one account, but reply comes on another

I have six e-mail accounts for different purposes. I am sending e-mails out on specific accounts and am finding that a response is coming from one of my other accounts. I know you can set a default e-mail account, and I have. But the e-mail that the responses are coming in on are not on the default account, nor on the account I sent it from. It makes absolutely no sense! And, is concerning.

Does anyone else have, or have had, this probem? If so, is it fixed? How? Help! Thank you!!