What Can Cause High Outgoing Email Traffic?

High outgoing email traffic means your website or mail server is sending a larger amount of data than normal. This does not always mean that spam or a security issue is occurring.

There are several common reasons why outgoing email traffic can become high.

1. Large Email Attachments

One of the most common causes is sending emails with large attachments.

Examples include:

  • Large PDF documents
  • Product catalogues
  • ZIP files
  • High-resolution images
  • Artwork or design files
  • Reports
  • Invoices with large attachments
  • Videos or other media files

Including forwarding email with large attachments

For example, sending a 20 MB PDF to 20 recipients can result in hundreds of megabytes of outbound traffic.
Email attachments are also encoded when sent, which can increase the amount of data transferred by approximately 33%.

2. Sending Emails to Multiple Recipients

An email with a large attachment sent to many recipients can generate significant traffic.
For example:
  • 25 MB attachment × 50 recipients = approximately 1.25 GB of data
If the email is sent individually to each recipient, the amount of traffic can be even more significant.

3. Automated Website Emails (Shopping Carts)

Websites can generate large numbers of emails automatically.
Examples include:
  • Order confirmations
  • Invoices
  • Shipping notifications
  • Password reset emails
  • Account notifications
  • Contact form notifications
  • Stock notifications
  • Abandoned-cart emails
  • System reports
If a website experiences a large increase in orders or activity, outgoing email traffic can increase accordingly.

4. Email Delivery Retries

When an email cannot be delivered, the mail server may automatically retry sending it.
Large emails that repeatedly fail delivery can therefore consume additional outbound traffic.
This can be particularly noticeable when sending large attachments to email addresses that are unavailable or temporarily rejecting messages.

5. Mailing Lists and Bulk Emails

Marketing emails, newsletters and other bulk communications can generate significant traffic.
A relatively small email can still generate substantial traffic when sent to thousands of recipients.

6. Compromised Email Account

A compromised email account may be used to send large quantities of spam.
This can result in:
  • Large numbers of outgoing emails
  • Increased mail server traffic
  • Mail queues becoming unusually large
  • The server's IP address potentially being flagged for spam
If an individual mailbox is responsible for an unusually high volume of email, the account should be investigated and its password changed.

Email Hosting with Fatgalah: We have a maximum outgoing email limit per mailbox, per hour. This is designed to help reduce the risk of a mailbox being compromised and sending large quantities of spam, which could result in the server's IP address or domain being blocked or blacklisted.

If you regularly need to send a large number of emails, please contact Fatgalah so we can discuss the most appropriate solution.


7. Compromised Website or Script

A website can also be compromised and used to send email.
This can occur through:
  • Vulnerable plugins
  • Outdated software
  • Compromised PHP scripts
  • Insecure contact forms
  • Stolen website credentials
In this situation, the email may not appear to have been sent from a normal mailbox.

8. Large Email Reports or Documents

Some websites and applications automatically generate documents and send them by email.
For example, an automated system may generate a PDF invoice or report for every order.
If the generated document is large, this can significantly increase outgoing traffic even though the number of emails being sent is normal.
What Should Be Checked?
When investigating high outgoing email traffic, we recommend checking:
  • The number of outgoing emails
  • The size of the emails being sent
  • Which mailbox or website is generating the emails
  • Whether large attachments are being used
  • Whether emails are being sent to large numbers of recipients
  • The mail server queue
  • Failed and repeatedly retried emails
  • Website scripts that send email
  • Recently installed or updated plugins
  • Whether any email accounts may have been compromised
High outgoing email traffic does not automatically indicate a problem. Large legitimate documents, automated emails and bulk communications can all cause significant increases in traffic.

If the traffic is unexpected, the first step is to identify which domain, mailbox or website is generating the traffic. This will usually make it much easier to determine whether the cause is legitimate email activity or something that requires further investigation.