diff --git a/nselib/smtp.lua b/nselib/smtp.lua
index 34c4740fa..169a455be 100644
--- a/nselib/smtp.lua
+++ b/nselib/smtp.lua
@@ -584,6 +584,17 @@ quit = function(socket)
   socket:close()
 end
 
+-- Attempts to authenticate with the SMTP server. The supported authentication
+--  mechanisms are: LOGIN, PLAIN, CRAM-MD5, DIGEST-MD5 and NTLM.
+--
+-- @param socket connected to server.
+-- @param username SMTP username.
+-- @param password SMTP password.
+-- @param mech Authentication mechanism.
+-- @return true on success, or false on failures.
+-- @return response returned by the SMTP server on success, or an
+--         error message on failures.
+
 login = function(socket, username, password, mech)
 	assert(mech == "LOGIN" or mech == "PLAIN" or mech == "CRAM-MD5" 
 			or mech == "DIGEST-MD5" or mech == "NTLM",