Exim dropping SMTP connections - "too many syntax or protocol errors" due to single quotes in RCPT TO
Hello community,
I am seeing repeated SMTP connection drops in my Exim log on a cPanel server (/var/log/exim_mainlog). The connection is dropped right after the RCPT TO command.
Here is an extract of the error log:
2026-09-17 16:24:50 SMTP connection from [190.236.yy.xxx]:56960 (TCP/IP connection count = 9)
2026-09-17 16:24:50 no host name found for IP address 190.236.yy.xxx
2026-09-17 16:24:51 SMTP call from (DESKTOPEJQPAEG) [190.236.yy.xxx]:56960 dropped: too many syntax or protocol errors (last command was "RCPT TO: <'user@domain.com'>", C=EHLO,AUTH,MAIL,RCPT,RCPT,RCPT,RCPT)
It seems the sending client (DESKTOPEJQPAEG) is incorrectly passing single quotes inside the angle brackets of the RCPT TO command (RCPT TO: <'user@domain.com'>), which violates RFC 5321 standard syntax and triggers Exim to drop the connection after multiple syntax errors.
My questions:
-
Is there a way in WHM / Exim Configuration Manager to temporarily relax protocol enforcement or sanitize/rewrite these malformed
RCPT TOaddresses via a custom ACL so the emails are delivered while the client fixes their software? -
Has anyone encountered a similar issue with specific mail clients or scripts adding literal quotes around the email address?
-
Over the years, yes I have encountered that. And my easy response was to tell the customer to send mail using something from the post-2000 era to actually send mail (meaning, use something that can reliably send mail and adhere to the rules). Just like the people who still try to run software that cannot SMTP authenticate and they expect you to whitelist the IP address of their whole buisness (exempt it from requiring SMTP Auth) just to work around their ancient software. I just explain that they have to bite the bullet and find something new that works properly.
1
Please sign in to leave a comment.
Comments
1 comment