smtp connection works in telnet but not in ASP -- same server, same credentials - asp.net

Question

I'm trying to send email from an ASP.NET using my SendGrid account. It works on my dev machine, but not in production, even though the credentials are the same. Likewise, in production I can connect to the SMTP server via telnet (using base64 encoded credentials), but the ASP site can't connect--I get error "Unauthenticated senders not allowed."
I've tried a mix of port numbers (25, 587, 465 -- my site is SSL). Using port 465 times out. 25 and 587 return respond immediately--but with the login error. This is really baffling because, like I say, it's the same credentials on dev machine and production.
I looked very briefly at Microsoft Network Monitor 3.4, but could not make heads or tails of it. I was hoping it would tell me the blow-by-blow commands being sent since I suspect the web site is doing something a little different from how telnet connects, but I don't know what.
Note I also asked my web host if outgoing traffic on these ports were blocked on production firewall, but they aren't.
Here's the actual code--like I say works fine on localhost, but SMTP connection fails in production
[AllowAnonymous]
[HttpPost]
public ActionResult ResetPasswordSend(string email)
{
List<string> userList = new List<string>();
try
{
string[] invalidChars = new string[] { ";", "," };
foreach (var invalidChar in invalidChars) if (email.Contains(invalidChar)) throw new Exception("Email contains invalid character.");
int count = 0;
// since emails are not unique, I must launch resets for all of them
var users = _db.HsProfile.Query("[Email]=#0", SqlDb.Params(email));
foreach (var profile in users)
{
count++;
userList.Add(profile.UserName);
var token = WebSecurity.GeneratePasswordResetToken(profile.UserName, 15);
WebMail.Send(profile.Email, "HumaneSolution.com Password Reset for user " + profile.UserName,
"You received this email because you or someone with your email address requested a password reset on HumaneSolution.com. " +
"If you didn't do this, then you don't need to take any action, and nothing will happen.\n\n" +
"To proceed with the password reset, click the link below within 15 minutes:\n\n" +
Url.BaseUrl("Account/EnterNewPassword/" + token) + "\n\n" +
"Sent to: " + email + " at " + DateTime.Now.ToLongDateString() + " " + DateTime.Now.ToLongTimeString() + "\n" +
"User name: " + profile.UserName);
}
if (count == 0) throw new Exception("Email " + email + " is not registered at HumaneSolution.com.");
}
catch (Exception exc)
{
ViewBag.Error = exc.Message;
}
return View(userList);
}

Answer 1

Based on suggestion from SendGrid, I re-wrote the email code so it does not use WebMail.Send but rather the SmtpClient and MailMessage objects explicitly. SendGrid says there might be some kind of timing problem in how ASP.NET loads credentials from the config file automatically. Here's exactly what they said:
Are you by chance storing your SendGrid credentials in a configuration file, separate from the code that connects to our SMTP server? The reason I ask is because I have seen rails and C# configurations like this receive the unauthenticated error due to the credentials not being passed at the correct time. Usually this is solved by moving the credentials directly in with the code instead of a separate configuration file. Give that a try and see if you notice a difference.<<
I didn't follow their advice completely -- i.e. I'm still using config file, but I'm loading the config values in subclasses of SmtpClient and MailMessage so I avoid hardcoding creds in my app. Anyway, it worked, all is well again.