public static string GetSAMAccountNameByEmail(string email)
{
    var oroot = new System.DirectoryServices.DirectoryEntry("LDAP://INVESTOR_AB");
    var osearcher = new System.DirectoryServices.DirectorySearcher(oroot);
    osearcher.Filter = string.Format("(&(mail={0}))", email);
    var oresult = osearcher.FindAll();

    if (oresult.Count == 0) throw new InvalidOperationException(string.Format("Cannot find mail {0} in LDAP.", email));
    if (oresult.Count > 1) throw new InvalidOperationException(string.Format("There are {0} items with mail {1} in LDAP.", oresult.Count, email));

    return oresult[0].Properties["sAMAccountName"][0].ToString();
}

Uses LDAP to find sAMAccountName by using the Email address.

Add System.DirectoryServices to references.

Get sAMAccountName by Email from LDAP

email

C#