CSF Firewall Exim Syntax Error

What causes the Exim Syntax Error?

This is generated on the user's side and often originates when using mail clients. The problem occurs when incorrect data is sent to the mail server from our IP, and the most frequent cause is usually with the recipient's email address. When we send an email through a local client (such as Outlook, Thunderbird, etc.) sometimes that program adds quotes to the address of the recipient ('[email protected]'). This is frequent when the receiver is not added to our contact list.

If we process the sending without removing those quotes, the mail server will take it as a syntax error and could block our IP. The problem is not only with the quotes but also with a series of characters that are not supported. Let's take a look at the following ones and see which ones are valid and those that are not.
Not valid:

  • The letter "ñ"
  • The accents
  • Characters other than periods or hyphens (example "$ =%)


  • Period (.)
  • Scripts (- and _)
  • All numbers (0 to 9)
  • All the letters, except the ñ (both lowercase and uppercase)

Blocks can also occur in the event that unrecognized characters are sent in the subject of the mail or in the body. Basically, any strange data that our mail client sends to the server can cause Exim Syntax Error. In most cases, the problem originates from typing the address of the recipient, so it is always advisable to check it twice before giving the send button.


When the block occurs, the fastest way to fix it is by contacting the hosting provider so that the IP is unlocked. Another possible solution is to restart our Internet device to release the and provisions a new IP will be automatically assigned. Now we know that certain characters are capable of causing the 'Exim Syntax Error', and the good thing is that we also know what are the methods to make that blocking no longer bother us.