Using Thunderbird POP3 SMTP Ubuntu 14.04 on my PC. Can't send messages but can receive. Error message is "4.4.0 ... Relaying temporarily denied. Cannot resolve PTR record for 126.96.36.199." Am only able to send messages to myself using ISP address.
Have reinstalled Thunderbird on PC and changed DSL router and spoken to ISP who can't see problem from their end but problem persists. Installed alternate email client Claws POP3 on same PC. Same problem as Thunderbird. Changed SMTP Port from 25 to 587, get different error message "mail server responded: 5.7.0 Authentication required"
Changed to laptop from PC and installed Thunderbird POP3 Ubuntu 12.04 on laptop which works perfectly. Seems that problem must be in Ubuntu/Thunderbird on my PC.
Appreciate any help. Thank you.