log4net is often used in administrative utilities and used to send mails in case of errors. It is total unrealistic to assume each used mail-address really exists. Mostly "virtual address" are used, means:They do not have any pop3 mailbox. So, if sending mail fails from the smpt server, no user can be informed. A property, which allows to specify the "reply-to" address on the SmtpAppender would solve this problem.