Mark may have more suggestions, but this is almost certainly the cause of the problem. The default for this parameter is anonymous_list_keep_headers: ^(?!x-) ^x-mailman- ^x-content-filtered-by: ^x-topics: ^x-ack: ^x-beenthere: ^x-list-administrivia: ^x-spam- which is a list of strings separated by spaces, and I believe the leading space on the second line is significant (meaning a continuation of the list on the first line). There are a number of possibilities that cause this. (1) Check whether anonymous_list_keep_headers is set in mailman.cfg. If it is, it may be all you need to do is remove a spurious newline after the keyword anonymous_list_keep_headers. Otherwise, you can restore the default by copying the setting above.
Thanks to this i found the cause of concern - my mailman.cfg had "empty anonymous_list_keep_headers:" option defined. I commented out and now test email went through by using anonymous option successfully. One small thing, but so much trouble. My bad that i didint notice it before. Big thanks again Steve and Mark!