I know that Yahoo and Google mail support 25MB email attachments. I have an idea from somewhere that 10MB email messages are generally supported by modern email systems.
So if I'm sending an email between two arbitrary users on the internet, what's the safe upper bound on message size? 1MB? 10MB? 25MB?
I know that one answer is 'don't send big emails, use some sort of drop box'. I'm looking for a guideline if you are limited to only using regular smtp email.
My guideline for unknown recipients is 5MB, but I try to keep it to 1MB if possible.
In three years at my last job, we did run into at least one or two cases where a ~5MB message was rejected, and we couldn't really get enough info from the recipient to troubleshoot more. Zipping the document got it through, and that satisfied the senior executive on our end.
Remember that not all recipients will have the knowledge, the patience, and/or the ability to determine if they have a size limit (or a block on certain extensions). So the larger the message, and the more arbitrary the other user is, the more likely you are to get caught in a morass.
I think this is going to be largely subjective. I work for an email hosting company and we allow 50MB attachements, because our customers demand that from us. I also do contract work for an insurance brokerage that doesn't impose any limit. You'll probably find most commercial email providers impose some limit, say between 1MB and 25MB, and most large corporations impose some limit, but finding the "average" is probably going to prove difficult.
As you can probably tell from the variety of answers posted here, usage patterns vary according to the profile of a mail servers' users. Rather than for a lack of a comprehensive study, usage patterns will differ significantly. Personal use often differs from business use and in any particular organisation, some users may have very different requirements than others. Guidelines should therefore consider the connection types of senders and recipients, for obvious reasons of latency of transmission, to make sure that network and other resources aren't abused.
As for a sensible an upper limit... in 2011, many people have a reasonably good broadband connection at home and businesses take advantage of broadband or better connection types. Space isn't much of an issue these days. But given the availability of different file transfer and file sharing options and social network sites, email isn't the best way to send a 50MB video file to 23 friends. Furthermore many people now use mobile email, so to help protect resource availability I set an upper limit of 15MB on mail servers I maintain.
