Some devices such as printers and fax machines send email message that do not comply with the SMTP RFC policy and IMail blocks that devices access to the server.

Open the Windows Registry editor located in %windir%\SysWOW64\regedt32.exe.

Create a DWORD value named AllowBareLF located in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\SMTPD32\Parameters\
Once changes are made to the registry restart the SMTP & Queue Manager Services for the change to take affect.

Value of 1 means it is enabled and will allow Bare LF.  Any other value indicates that the value is disabled and will reject messages. 

To confirm that the change is working open your SMTP log and look for the following line.

Allow Bare LF: True

Once you see that value your devices will now be able to send messages.