I suppose there is some banner setting in the FTP server, I too faced the same issue. The response might change based on the settings in the FTP server across environments(DEV,TEST,PROD), so I would suggest you to use double checking in your before enter the processing sequence.
- Check for dirList is not null, then
- always check for the first element is NOT EQUAL to “550 TAU*.txt: No such file or directory”.
If these conditions are met then that means there is a valid file available for processing. This is what I did as then I was running on tight timelines and I didnt have time to investigate on this so did a work around. I will monitor this post for any solution from others.