If the error is caused by the user, the message should not apologize. Sorry, the email address you entered appears to be invalid. That just sounds condescendi

If the error is caused by the user, the message should not apologize. Sorry, the email address you entered appears to be invalid. That just sounds condescending; you know they're not really sorry. However, if the error is caused by your system, including any third party dependencies, then an apology is in order. Sorry, the requested service appears to be inaccessible at the moment. Please try again later.