Guidelines on Email Size

Asked on the REACH List: Can anyone give me guidelines as to email attachment size limits? Within UIC domain and outside as well.

Reply:Short answer: The guideline is "Keep them as small as possible." 8-)

Longer answer: The limits we have in place are applicable to the message as a whole, rather than attachments alone. Enforcement occurs during the SMTP connection that transfers mail from one machine to another -- for example, from a machine outside UIC to tigger or from your office machine running Eudora to smtpserv1.cc.uic.edu.

This means that if you're sending mail outside UIC, barring any limits imposed by your mail client itself, it's up to the mail relay that receives your mail to impose a size limit. Note that many mail relays may handle your mail before it reaches the final recipient, and each one can have its own limits. Also LISTSERV lists may have size limits imposed by their owners.

Our machines will reject individual pieces of mail over 1 MB (approximately) for icarus and 10 MB for eeyore and tigger. [Eeyore is an ADN service machine; one of its tasks is handling email sent to uic.edu.] There are some cases in which mail will not be rejected even if it is above this limit.

For comparison, a rough calculation shows that over a PPP link running at 3 KB per second, which I sometimes experience over a modem connection, a 1 MB message would take in the neighborhood of five minutes to download. I bring this up because if you want a "guideline" for message size, this is one way of looking at it -- i.e. consider what recipients with slow Internet connectivity will think about it.

Another consideration is the resources used to transfer the mail. Larger messages require more storage space and CPU time on the mail relays that process them. Sending the message to many recipients amplifies this.

Finally, it's hard to define what "large" and "small" messages are because it's subjective. I personally don't like to see anything larger than 20 KB, but that's just me. Some of the complaints I've seen in the past were not primarily about the size of an attachment or message, but because the sender did not consider who their audience was.

Rather than expend any more bytes on this, I know that you can find Web sites and books about email etiquette.