AUTH PLAIN for SMTP is not working on all servers

[paresy]

I did this

curl smtp://smtp.world4you.com --user "username:password" -v --login-options
 AUTH=PLAIN

Output:

* Rebuilt URL to: smtp://smtp.world4you.com/
*   Trying 81.19.149.200...
* TCP_NODELAY set
* Connected to smtp.world4you.com (81.19.149.200) port 25 (#0)
< 220 mx01lb.world4you.com ESMTP Exim 4.92 Wed, 26 Jun 2019 13:42:29 +0200
> EHLO DESKTOP-E0U2VS3
< 250-mx01lb.world4you.com Hello DESKTOP-E0U2VS3 [212.72.166.112]
< 250-SIZE 157286400
< 250-8BITMIME
< 250-PIPELINING
< 250-AUTH PLAIN LOGIN
< 250-CHUNKING
< 250-STARTTLS
< 250 HELP
> AUTH PLAIN
< 535 Incorrect authentication data
* Closing connection 0
curl: (67) Login denied

I expected the following

> AUTH PLAIN ***base64encoded username/password here***
< 235 Authentication succeeded

Further information

AUTH LOGIN works fine.
AUTH PLAIN with GMail works fine aswell.

If you need credentials for the mentioned mail server i can provide some.
Using Telnet and manually sending the commands with AUTH PLAIN works as intended.

Using GMail the following happens:

> AUTH PLAIN
< 334
> xxxxxxxxxxxxxxxxxxxxxxxx

There seems to be an additional 334 response which the world4you server does not send. Is this allowed behavior by the SMTP specs? Should libcurl be able to handle this scenario?

curl/libcurl version

curl 7.58.0 (x86_64-pc-linux-gnu) libcurl/7.58.0 OpenSSL/1.1.0g zlib/1.2.11 libidn2/2.0.4 libpsl/0.19.1 (+libidn2/2.0.4) nghttp2/1.30.0 librtmp/2.3
Release-Date: 2018-01-24
Protocols: dict file ftp ftps gopher http https imap imaps ldap ldaps pop3 pop3s rtmp rtsp smb smbs smtp smtps telnet tftp
Features: AsynchDNS IDN IPv6 Largefile GSS-API Kerberos SPNEGO NTLM NTLM_WB SSL libz TLS-SRP HTTP2 UnixSockets HTTPS-proxy PSL

I also tried on the curl 7.64.1 which has the same issue.

Thank you for reading!

[bagder]

It appears that when issuing that command line, "AUTH PLAIN" is followed by a CRLF and then the base64 encoded blurb comes... @captain-caveman2k might have some clues here?

I tried a similar command line against an SMTP server of mine (using curl from git master) and it did this:

$ curl smtp://[server] --user foo:bar -v --login-options AUTH=PLAIN
...
< 250-STARTTLS
< 250-DELIVERBY
< 250 HELP
> AUTH PLAIN
< 334 
> AGZvbwBiYXI=

[bagder]

Seems nobody wants or can work on this issue. It will be added as a known issue and closed within soon unless something changes.