In the BTMail system, a red exclamation mark means it has been marked by the sender as a "high priority" email. This is also usually the case if the sender is using some other email service such as gmail.
How are you accessing your email account. Is it with an email client/app or are you using a web browser?
Is the email from a known and trusted contact?
Have you had emails from the sender prior to the one you are mentioning here?
Are you using the "reply to" function or are you entering the email address that you are replying to manually?
When you say "When I reply or send a new email to the address they appear not to have received it". How do you know the have not received it?
Are you sure that it is not a spam email?
