C:\>curl --version curl 8.21.0-DEV (Windows) libcurl/8.21.0-DEV Schannel Release-Date: [unreleased] Protocols: smtp smtps Features: AsynchDNS IPv6 Kerberos Largefile SPNEGO SSL SSPI threadsafe Unicode UnixSockets C:\>curl --url smtp://smtp.acme.com:587 --mail-from test@acme.com --mail-rcpt test@acme.com --upload-file test.txt -v --negotiate -u : * Uses proxy env variable no_proxy == '192.181.0.51' * Host smtp.acme.com:587 was resolved. * IPv6: (none) * IPv4: 192.181.0.52 * Trying 192.181.0.52:587... * Established connection to smtp.acme.com:587 (192.181.0.52 port 587) from 192.181.0.50 port 50882 % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 0 0 0 0 0 0 0 0 0< 220 smtp.acme.com:587 Microsoft ESMTP MAIL Service ready at Fri, 15 May 2026 07:47:03 -0700 > EHLO test.txt < 250-smtp.acme.com:587 Hello [192.181.0.50] < 250-SIZE 37748736 < 250-PIPELINING < 250-DSN < 250-ENHANCEDSTATUSCODES < 250-STARTTLS < 250-AUTH GSSAPI NTLM < 250-8BITMIME < 250-BINARYMIME < 250-CHUNKING < 250 SMTPUTF8 > MAIL FROM: SIZE=164 0 0 0 0 0 0 0 0 00:04 0 < 530 5.7.57 SMTP; Client was not authenticated to send anonymous mail during MAIL FROM * MAIL failed: 530 * shutting down connection #0 curl: (55) MAIL failed: 530