livemail.co.uk SMTP bug

Recently one of our clients reported an issue with the SMTP server they use. The problem occurred only in Mail.dll. Outlook was working correctly.

The exception we are getting is:

Limilabs.Client.ServerException: 5.5.1 Invalid command
at Limilabs.Client.SMTP.Smtp.[1](String [1], Boolean
)
at Limilabs.Client.SMTP.Smtp.LoginDIGEST(String user, String password)
at Limilabs.Client.SMTP.Smtp.UseBestLogin(String user, String password)
at ..Forms.ComposeEmailForm.SendEmail()

The SMPT settings are as follows:

Outgoing mail server: smtp.livemail.co.uk
Port: 587
Use SSL: false

After some investigation it turned out that livemail.co.uk incorrectly advertises CRAM-MD5 login method:

The server EHLO response is misleading:


S: 250-AUTH LOGIN PLAIN CRAM-MD5 DIGEST-MD5
S: 250-ENHANCEDSTATUSCODES
S: 250-8BITMIME
S: 250-DSN
S: 250 AUTH PLAIN LOGIN CRAM-MD5

Notice that it sends DIGEST-MD5 in first AUTH response and it doesn’t send it in the second one. Unfortunately Mail.dll uses the first response and it causes it to use CRAM:

C: AUTH DIGEST-MD5

the command fails, because it’s most likely not implemented by the server or not turned on:

S: 500 5.5.1 Invalid command

AUTH PLAIN and AUTH LOGIN methods do work without any problems

using (Smtp client = new Smtp())
{
    client.Connect("smtp.livemail.co.uk");
    client.Login("user", "pass");
    //...
    client.Close();
}

using (Smtp client = new Smtp())
{
    client.Connect("smtp.livemail.co.uk");
    client.LoginPLAIN("user", "pass");
    //...
    client.Close();
}