How To Resolve Maximum Line Length For SMTP Transports Error In cPanel

Symptoms

• Emails bounce with obscure SMTP errors
• Clients report "message too long" or "line length exceeded"
• Your transactional emails (like property inquiries) go missing

Possible Cause

The Maximum line length for SMTP transports setting in cPanel controls how long email lines can be before your server refuses to send them. The SMTP protocol limits line length to 1000 characters including CRLF. This is an international standard (The RFC 5322 (SMTP 5321)). By default, cPanel uses 2048. This is non-compliant and violates RFC 5321 standards as values like 2048 can be used when you're delivering internally or to non-standard MTAs.

• RFC 5321 Standard: Maximum 1000 characters per line (including CRLF)
• cPanel Default: 2048 characters (violates standards!)
• Real-World Impact: Remote servers reject your emails as non-compliant

RFC 5321 998 Characters

        998 characters for content
        + 2 characters for CRLF (\r\n)
        = 1000 total (RFC 5321 compliant)
      

Why cPanel Uses 2048 for SMTP Line Length

As stated above, RFC 5321 (the SMTP standard) dictates a maximum line length of 1000 characters, including CRLF. In practice, this means the body of each line must not exceed 998 characters before the line break.

While cPanel's choice to set the default SMTP line length to 2048 may appear to violate standards, but it's a calculated decision driven by real-world email behavior and user experience.

Why Does cPanel Use 2048?

• Accommodating Modern Email Clients
  

  Many popular email clients (like Outlook, Thunderbird, and webmail apps) generate Base64-encoded messages or HTML bodies that frequently exceed 998 characters per line. Setting a limit of 2048 avoids delivery errors or silent failures in these cases.

• Minimizing Support Tickets
  

  Most cPanel users are not sysadmins. They use CMS tools (like WordPress) that send complex emails without granular control over MIME formatting or line breaks. Rather than frustrate users with cryptic SMTP rejections, cPanel prioritizes resilience over strict compliance.

• Compatibility vs. Purism
  

  In the evolving landscape of email delivery, well, many modern mail systems tolerate longer lines without rejection. cPanel makes a conscious trade-off which is to prioritize successful delivery in 99% of cases over strict RFC purity.

What about Plesk? How Does Plesk Handle This?

Plesk doesn't directly set the SMTP line length in its GUI, but it typically ships with Postfix as its MTA.

Surprisingly, Postfix defaults to a message_line_length_limit of:

      postconf | grep line_length_limit
      

Output is usually:

  line_length_limit = 2048
  lmtp_line_length_limit = 998
  smtp_line_length_limit = 998
      

This means that Postfix is kinda sticking to strict standards.

SMTP Line Length Comparison Table

Solution for Regular cPanel Users

• Switch to Webmail instead of Outlook or Thunderbird. This respects server line length limits automatically. Perfect for urgent emails while your host fixes the issue
• Try the latest version of Microsoft Edge or Safari. Configure Thunderbird with different line wrapping settings. See Content conversion in Exchange Server for more info.
• Use plain-text format temporarily instead of HTML emails as clients like Outlook may automatically format content into longer lines. Note that Base64 and MIME attachments are especially sensitive to line lengths.
• Contact our technical team by submitting a support ticket requesting RFC 5321 compliance. Mention potential delivery issues with major email providers.

Solution for Root Users

Diagnose Your Current Configuration

Before fixing anything, it is important to see what you're working with. This command checks all SMTP transport configurations:

      sudo exim -bP transport_list | while read list; do echo "$list"; exim -bP transport $list | grep line; done
      

This command checks all transports (custom, remote_smtp, etc.) for their message_linelength_limit.

• exim -bP transport_list - Lists all email transport methods
• while read list - Loops through each transport
• grep line - Shows line length settings for each transport

You'll likely see output like:

        message_linelength_limit = 2048
        dkim_remote_smtp
        message_linelength_limit = 2048
        remote_forwarded_smtp
        message_linelength_limit = 2048
        dkim_remote_forwarded_smtp
        message_linelength_limit = 2048
        -- truncated ---
      

You can also use:

sudo exim -bP transport remote_smtp | grep message_linelength_limit

Method 1: WHM Interface (Recommended)

• Log into WHM as the 'root' user
• Go to: WHM » Service Configuration » Exim Configuration Manager
• Click on the "Mail" tab
• Find "Maximum line length for SMTP transports"
• Change the value from 2048 to a higher value.
• Click "Save"

Method 2: Advanced Editor (TRANSPORTSTART Section)

Always backup your Exim configuration before changes and keep a record of your modifications for future reference.

1. In WHM: Go to Exim Configuration Manager » Advanced Editor
2. Find Section: TRANSPORTSTART
3. Add these lines at the end:

        smtp:
          driver = smtp
          port = 587
          hosts_require_auth = *
          hosts_require_tls = *
          message_linelength_limit = $value
      

Email and Your Business

Email reliability isn't just technical, it's business critical. A reliable email delivery maintains professional credibility, and ensures order confirmations and invoices reach customers.

If you want an email that just works, try our business-class email hosting optimized for:

• Business professionals and security conscious users
• High-volume lead forms and newsletters
• Bulletproof deliverability to Gmail, Outlook, and Yahoo

You can start with our normal existing email packages, or consult with our team to deploy a well-configured, security hardened cPanel Email Only server, or use one of our 1-click Webmail Lite, Roundcube and/or Postfix Admin email applications to host your email.

We also provide full email integration services if you are using Google Workspace, Proton Mail, Apple Mail, etc.