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Email Attachment Policies

Attachments are one of the most useful features of email. The general public uses email attachments to share photos with friends and family, businesses use them to exchange legal documents, and, in an emergency situation, a government could use attachments to convey important safety information. In a natural disaster, for example, if the government’s Web site was unavailable, an email containing a file with important safety tips could be very valuable.

Like most people, I tend to take attachments for granted; however, two recent questions caused me to examine attachments a bit more closely. A client wanted to know if email bounce rates were higher for emails with attachments, compared to emails without them. Another client wanted to know if there was general data about businesses blocking attachments from their email servers. While I couldn’t find an answer to the second question, finding an answer to the first one was eye opening, and sheds some light on the second.

Nearly 50M emails are sent using GovDelivery each month. That data provides access to a lot of valuable data about how ISPs treat attachments. We examined the data and concluded that while senders who include attachments have slightly higher bounce rates, the differences are so small as to be almost negligible, far less than 1%. Getting to this answer uncovered some interesting details, which I’d like to share here.

The major email providers, including AOL, Gmail and Yahoo! have fairly liberal policies regarding attachments, even accepting rather large files, or more accurately large message sizes. (A message could include multiple files. The sum of the attachments and the email content makes up the message size.)

  • AOL 16MB
  • Gmail 20MB
  • Yahoo! 10MB (a premium edition supports larger file sizes)

The primary concern with email attachments for most ISPs and mail recipients is the risk of viruses. AOL’s statement about attachments is clear on this point: “Attached files can contain virus programs or Trojan Horse programs that could damage the files on your computer or steal your password. Never download a file unless you know the person who sent you the file, and were expecting to receive that particular file from him/her. ”

To protect their customers, AOL, Gmail and other providers block emails that contain attachments of some file types. Gmail’s policy is “Gmail won’t accept these types of files even if they are sent in a zipped (.zip, .tar, .tgz, .taz, .z, .gz) format. If this type of message is sent to your Gmail address, it is bounced back to the sender automatically.”

Some other things to consider if you plan to send attachments, especially large attachments, regardless of the end-user’s email provider is do they use dial-up and other slow Internet connections? These connections can prevent attachments from downloading completely. Other reasons a user might not receive your attachment include:

  • Full browser cache will prevent attachments from being downloaded
  • Full email box prevents delivery of the attachment. Email providers are expanding size limits all the time, but corporate and small ISPs may still have smaller limits.

Of the 50M emails sent through GovDelivery each month without attachments, 99.57% successfully reach their final destination. The remaining 1/2-percent are sent to invalid email addresses, defunct ISPs, or are filtered by overzealous filtering applications. Emails with attachments, reach their destination at only a slightly lower rate overall, 99.20%. GovDelivery allows users to send emails up to 1MB in size, including message content. Also, GovDelivery will accept any number of attachments, as long as the sum is less than 1MB. The limit for linked file attachments is 10MB. Judging by the delivery rates, this is sufficient to pass most ISP filters.

If you are concerned about your message getting through to the end user, a common alternative to an attachment is to post the content you’d normally include in the email to a Web page. Provide a link to the URL in the email. Other strategies include compressing large or multiple attachments using a program like WinZip. Lastly, always remember that common file types are best. Adobe Reader is free, so sending a PDF is a decent way to ensure that your message can be read, other file types may require expensive programs to run.

Attachments are still a useful and common part of email communication. Taking a few precautions will help ensure that the messages reach their final destination.