What might cause this IMAP client error - "'Unexpected: Non-last response line..."

[rlebeau]

The error happens during the execution of TidImap4.UIDRetrievePeek, any hint as to trace the source of the issue?

Not without seeing the complete IMAP response that was received from the peek command. But, the error basically means that the response contained at least 1 line of text that did not start with a leading '*' character, and that line was not the last line in the response. UIDRetrievePeek() sends a FETCH command, and every line in the response, except the last, should begin with "* # FETCH ..." (where '#' is an email message number), and the last line contains the final status code for the response (OK, NO, BAD).