How to interpret CURLE_WEIRD_SERVER_REPLY?

From: SherwoodP via curl-library <curl-library_at_lists.haxx.se>
Date: Sat, 18 Oct 2025 11:11:18 -0700

BACKGROUND:
I am trying to use curl v8.6.0 on a Windows 11 system to send email to my POP3 server (att.com). AT&T uses "secure mail keys" for authentication.

I can access AT&T's email web interface using a user name and password.
I can also access the same account with Eudora, which requires a secure mail key.

USING CURL:
This is the curl C++20 code I am using to send a sample message:

curl = curl_easy_init();
if (curl)
{
CURLcode code=curl_easy_setopt(curl, CURLOPT_URL, si .server);
code=curl_easy_setopt(curl, CURLOPT_USE_SSL, (long ) CURLUSESSL_ALL );
code = curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 0L); // Tell libcurl to *not* verify the peer
code = curl_easy_setopt(curl, CURLOPT_USERNAME, si .userName);
code = curl_easy_setopt(curl, CURLOPT_PASSWORD, si .pw);
code = curl_easy_setopt(curl, CURLOPT_MAIL_FROM, si .from);
recipients = curl_slist_append(recipients, si .to);
if (strlen( si .cc)) // cc: is optional
    recipients = curl_slist_append(recipients, si .cc);
code = curl_easy_setopt(curl, CURLOPT_MAIL_RCPT, recipients);
code = curl_easy_setopt(curl, CURLOPT_READFUNCTION, payload_source);
code = curl_easy_setopt(curl, CURLOPT_READDATA, &upload_ctx);
code = curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);

/* send the message (including headers) */
res = curl_easy_perform(curl);
}

Structure si at the beginning of the code above contains
si{
server = "smtp://outbound.att.net:587"
userName = "MyUserName"
pw = "MySecureKey"
t = "ToEmailAddr"
from = "FromEmailAddr"
cc = "<info_at_example.org>"
messageID = "Communicate_000000006334"
subject = "Communicate test email"
nBodyLines = 10
body
[0] = "\tThe Germ"
[1] = ""
[2] = "A mighty creature is the germ,"
[3] = "Though smaller than a pachyderm."
[4] = "His customary dwelling place"
[5] = "Is deep within the human race."
[6] = "His childish pride he often pleases"
[7] = "By giving people strange diseases."
[8] = "Do you, my poppet, feel infirm?"
[9] = "You probably contain a germ."
};

Running this code gives res = CURLE_WEIRD_SERVER_REPLY (8) "curl_easy_perform() failed: Weird server reply" if I use the AT&T secure key as si.pw, and "Login denied" if I use the AT&T password.

To debug my usage of curl, I'd like to know what the "weird reply" is, or any other logging that curl provides. If the curl code is wrong in any way, or missing some detail of authentication, I'd appreciate advice on how to fix it.

For comparison, the email client Eudora can successfully send email to this account. An excerpt from Eudora's debugging log is below. This shows that Eudora has successfully logged in.

MAIN    16:14.54 Preparing messages to Send: 1
MAIN    16:14.54 RSET
MAIN    16:14.54 MAIL FROM:<FromEmailAddr>
MAIN    16:14.54 RCPT TO:<ToEmailAddr>
MAIN    16:14.54 DATA
MAIN    16:14.54 Test message
MAIN    16:14.54 Preparing messages to Send: 0
21296    16:14.54 Open 67.195.12.35:465
21296    32:14.54 Sent: " 16 03 01 00 F4 01 00 00 F0 03 03 C5 93 55 57 E6"
...
21296    64:14.54 Rcvd: " 16 03 03 13 2E 02 00"
...
21296    64:14.54 Rcvd: "220 smtp.mail.yahoo.com ESMTP ready\r\n"
21296    32:14.54 Sent: "EHLO MyComputerName.MyEmailDomain\r\n"
21296    64:14.54 Rcvd: "250-hermes--production-gq1-6b8576c5cf-gs5mr Hello MyComputerName.MyEmailDomain [xx.xx.xx.xx])\r\n"
21296    64:14.54 Rcvd: "250-PIPELINING\r\n"
21296    64:14.54 Rcvd: "250-ENHANCEDSTATUSCODES\r\n"
21296    64:14.54 Rcvd: "250-8BITMIME\r\n"
21296    64:14.54 Rcvd: "250-SIZE 41697280\r\n"
21296    64:14.54 Rcvd: "250 AUTH PLAIN LOGIN XOAUTH2 OAUTHBEARER\r\n"
21296    32:14.54 Sent: "AUTH LOGIN\r\n"
21296    64:14.54 Rcvd: "334 xxxxxxxxxxxx\r\n"
21296    32:14.54 Sent: "xxxxxxxxxxxxxxxxxxxxxxxxxxxx\r\n"
21296    64:14.54 Rcvd: "334 xxxxxxxxxxxx\r\n"
21296    32:14.54 Sent: "xxxxxxxxxxxxxxxxxxxxxxxx\r\n"
21296    64:14.56 Rcvd: "235 2.7.0 Authentication successful\r\n"
21296    32:14.56 Sent: "RSET\r\n"
21296    64:14.56 Rcvd: "250 2.0.0 OK\r\n"
21296    32:14.56 Sent: "MAIL FROM:<FromEmailAddr>\r\n"
21296    64:14.56 Rcvd: "250 2.1.0 Sender <FromEmailAddr> OK\r\n"
21296    32:14.56 Sent: "RCPT TO:<ToEmailAddr>\r\n"
21296    64:14.56 Rcvd: "250 2.1.5 Recipient <ToEmailAddr> OK\r\n"
21296    32:14.56 Sent: "DATA\r\n"
21296    64:14.56 Rcvd: "354 Ok Send data ending with <CRLF>.<CRLF>\r\n"
21296    32:14.56 Sent: "X-Mailer: QUALCOMM Windows Eudora Version 7.1.0.9\r\n"
21296    32:14.56 Sent: "Date: Sat, 18 Oct 2025 10:53:07 -0700\r\n"
21296    32:14.56 Sent: "To: ToEmailAddr\r\n"
21296    32:14.56 Sent: "From: FromEmailAddr\r\n"
21296    32:14.56 Sent: "Subject: Test message\r\n"
21296    32:14.56 Sent: "Mime-Version: 1.0\r\n"
21296    32:14.56 Sent: "Content-Type: text/plain; charset="us-ascii"; format=flowed\r\n"
21296    32:14.56 Sent: "\r\n"
21296    32:14.56 Sent: "Body of message.\r\n"
21296    32:14.56 Sent: "\r\n"
21296    32:14.56 Sent: ".\r\n"
21296    64:14.57 Rcvd: "250 OK , completed\r\n"
21296    32:14.57 Sent: "QUIT\r\n"
21296    64:14.57 Rcvd: "221 Service Closing transmission\r\n"

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Received on 2025-10-18