Can forward email but cannot send directly...

I have trouble sending an e-mail to a few addresses. I can receive mail from
them but when I "reply" to the email I get an error message. Some new
addresses also will not send but I can forward the sam email to the same
address from the sent list/column/file. If this happened once or twice I
would guess that I fat fingered the address while typing but I review the
typed address in the error message (it lists the address in the error
message) and it the same. I forwad to the same address and it process
nicely. Help.