There is no problem with SmartFTP. The only problem is with your FTP server and probably with your understanding of the situation.
I can certainly relate to the frustration of dealing with users who have a poor or an incorrect understanding of a particular issue, and I imagine at some points it is difficult to deal with the constant "whining" from a user community, but I disagree with the above statement.
It is true that the origin of the particular problem SmartFTP users are experiencing may be that the Size Check on the server fails, but seeing as how in previous versions of SmartFTP this size check was never an issue, the problem is with SmartFTP's behavior when a file size mismatch is encountered on a SIZE check.
What I, and I am sure many other users would appreciate would be an option to have SmartFTP ignore the Size check and continue uploading files instead of stopping when the first file size mismatch is encountered.
After the above reply I am now left thinking that SmartFTP takes a "not my problem" approach to any users who are experiencing leigitimate problems that aren't necessarily SmartFTP's issue and while these problems may originate with other software there are certainly ways for SmartFTP to give users options of dealing with these kinds of problems, even if that means disabling features like a size check.
I beleive SmartFTP has to be, without a doubt, the best FTP client available, but replies like the one above could definately turn users away from your product.