Inexplicable change of email address
This is probably connected with a "reply to"-address provided with the email you received. For example, many companies use this when sending automatic generated e-mail from an address that doesn't receive e-mails. When you reply to such mail, your application checks if there is a "reply to"-address provided in the message's head, and if it is, it will choose that for your reply over the senders address. If your mail was not from a company but from a friend, this could be a bug or mistake in his mail application, maybe he provided such a reply-to-address to. Some applications do this when a feature to collect all mail in a single inbox is chosen. If none of these apply, you should check the authenticity of the sender's address, and maybe talk to him on the mail you received, as this could also be a sign of spam messages.