Sender domain "invalid"

[Stein Ma]

Dear all,     I am new to qmail and have a problem here.     I have added the follwoing in the .bash_profile     MAILUSER=my_isp_username     MAILHOST=my_isp_domain     QMAILINJECT=f     export MAILUSER MAILHOST QMAILINJECT   Thank you, Stein Ma   ===== qmail bounced the following messages ====   Hi. This is the qmail-send program at durian.fidamy.com. I'm afraid I wasn't able to deliver your message to the following addresses. This is a permanent error; I've given up. Sorry it didn't work out. <[EMAIL PROTECTED]>: Connected to 208.230.135.220 but sender was rejected. Remote host said: 501 <[EMAIL PROTECTED]>... Sender domain must exist ; ; ;