For those who use the Thunderbird mail client, I'm wondering why this message pops up every time I try to send an outgoing message. I'm accessing my university e-mail accounts by accessing their servers as IMAP servers. Here is the error message I get after clicking 'send.' An error occurred while sending mail. The mail server responded: 5.7.1 Rejected 18.104.22.168 found in dnsbl.sorbs.net. Please verify that your email address is correct in your Mail preferences and try again. Any help with this would be appreciated.