Thanks for the explanation. We have a similar function on our Canon copier. But while we enter smb://computer/share on the Mac, on the Canon we need to enter the path as \\computername\share.
The other thing to note is that you have may have an issue with IP address to NetBIOS name resolve. In other words, the Lexmark may not be able to get the computer name RMR3 resolved to an IP address. So if you use the IP address instead of the computer name, such as \\ip_address\share, then this may also help.
The last thing to mention is that the Lexmark appears to be using the SMB syntax. And the SMB protocol in Lion has changed and is no longer compatible with a number of multifunction copiers/printers. So even if you do get the correct syntax, you get errors when you try to scan stating that your user credentials are incorrect - which is not the case. If you are not scanning to a Lion Mac but to a Windows computer, then this SMB compatibility is not going to be the issue.