Email Attachment Size Limits

Given I have plenty of disk space to work with, what are some technical considerations that I need to consider when configuring email attachment size limits?

Some of the considerations:

• Mail limits on other email systems, not everyone likes receiving 150MB attachments. Bounces will annoy YOUR users (and consume their quota, if any), and generate calls.
• Any mailbox size quotas you have in place. Large attachments make hitting them a LOT easier.
• Large-attachment virus scanning overhead.
• If your mailer is database backed (Exchange, others) how large of a database you're willing to put up with.
• How long it takes to recover from disaster. A large mail-store can take a lot longer than smaller ones.
• How many users you have and however long it takes to hit whatever mail-quotas you have in place.
• Mail client mail-download times(1). If a user has a 15GB mailbox, even in IMAP some clients will pull down all the mail.
• Mail client mail-download times(2). 75MB attachments can take a really long time to pull from your mail-server. This may annoy users.

I have a 10MB attachment limit set on my Exchange server for the following reasons.

First of all, those huge 20MB PowerPoint presentations that people send around are taking up space on my server which would be better placed on a network drive/in SharePoint (this isn't necessarily an issue if your mail server only delivers mail rather than storing it too).
Disk space may well be plentiful on your server, but email is not designed as a method of transferring files to co-workers, neither is it designed for storing masses of attachments. You need to think of the time it will take to recover all this information if your email server goes pop and you have to restore from backup. That extra 2GB of attachments in peoples mailboxes takes precious time to restore, and in my experience Exchange isn't the quickest at restoring from backup.

As I mentioned above, and so did jscott in his comment - email is NOT a method for your users to transfer files to each other. This should also be done via some kind of shared network drive, USB Hard Disk or USB memory sticks.

If these attachments are going external to your company, that huge attachment has to go through the internet. If you have a slow connection from your server to the outside world, this will take time to send and if you have a bandwidth limit on your line, you will very quickly eat into that. It will also take an equally long time for the receiving party to download it too.

Please also think of the recipient when you send large emails. They also may be on a slow connection (more than likely if they are on a home grade ADSL) and won't appreciate it when they press Send/Receive and it takes forever for their new mails to download because of a large attachment. Equally, they will probably have a bandwidth limit (especially so if it's a home user) and would probably be rather angry if your large email took them over their bandwidth limit and they were charged for it.

There are also factors beyond your control. When an email is sent across the internet, it may pass through many mail servers. These intermediate mail servers may have a size limit imposed on them. If that happens, your message won't be delivered to the recipient because it is too large.

If you are meaning for incoming mail and/or internal, then space is the only real factor unless there are store size problems with your mail system (if you specify what mail system you are using people may be able to give you specific hints there) - though make sure your users know there are often better ways to distribute large content and make sure you consider the implications for your backup solution.

For outgoing mail you should set a reasonable limit otherwise people may send out massive mails which go out (taking bandwidth and wasting their time) only to be rejected at the other end for being over-sized. Many mail systems set the maximum size (after any encoding of binary data, like base64) to around 10Mbyte which I think is fairly sensible with today's connection speeds though smaller limits are not uncommon. A rejection at your end will be much quicker than a bounce from outside.