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Emailing attachments, particularly large files, is getting more and more difficult as ISPs limit size and scan for spam and viruses.

I have noticed that a number of my emails w/attachments have not been delivered. One ISP actually told me the email was not delivered. I divided it into 2 emails w/attachments of less than 100kb and it seemed to be delivered. Could this be my problem (I have and pay for Hotmail and receive large files up to 20mb regularly) or a problem with my correspondent's email account?

There are many, many, many reasons that emails with attachments often fail to get delivered. There are a lot of potential problems in the scenario you outline, and I'll try and touch on a few of them.

We can start by blaming viruses.

Here are just some of the most common reasons legitimate emails with attachments may not make it through.

  • Spam/Virus Filters: perhaps the largest issue is that many viruses that attempt to send themselves in email as attachments. As a result ISPs, mail programs and mail servers have all had to become extremely suspicious of any and all attachments.

    "... email from any of the free email services ... also carries with it the reputation of that service."

    As should you be suspicious. But you have the advantage of knowing whether or not you can trust the sender, or even whether or not you're expecting an attachment from a particular person. Spam and anti-virus filters have no such knowledge.

    So an attachment counts against you when the recipient's spam filter analyzes your incoming mail. If it has an attachment, it's somewhat more likely to be considered spam, and filtered or blocked accordingly.

  • Reputation: people are going to claim I'm bashing Hotmail again, but the fact is sending email from any of the free email services (even though you're paying for it) also carries with it the reputation of that service. When it comes to spam, Hotmail and the others do not have a good reputation; spammers use and abuse them all the time. That reputation is something that spam filters pay attention to.

    The fact that your email comes from something also counts against you, and means that it's more likely to be flagged as spam. Add the fact that an attachment - a large attachment - also counts against you, and that's two strikes against your email getting delivered.

  • Attachment Blockers: for the very same reason that mail systems are looking at attachments as a bad thing, many email programs now come pre-configured to actually block access to some or all attachments. The recipient must somehow indicate that attachments are alright. Depending on their email program, this may be something that they can configure by file type (allow ".jpg" attachments, but not ".exe"), by sender (usually by adding the sender's email address to an address book or contact list), or globally.

  • Size Matters: as the internet slowly gets faster and faster many consider this as becoming less and less of an issue, but in fact we're keeping up by having and trying to transfer larger and larger files. The fact is large files always have been a problem, and ISPs regularly disallow emails that exceed a certain size. The problem, of course, is that even though "a certain size" might be getting larger over time, it will also vary from ISP to ISP, if they do this. Thus even if your ISP allows you to send a large file, that doesn't mean that your recipient's ISP will allow him to receive it.

    Larger files are an issue because of the time it takes to send them, and the disk space required to hold them on the multiple servers they traverse as they make their way from point A to point B. Unlike simply copying a file from one machine to another, where only two machines are impacted, email travels across several servers and each of those must have the space to hold your files and an internet connection fast enough to receive and transmit it in a reasonable amount of time, along with everything else that server might be doing.

There are more reasons, but those are the biggies.

Your next question, of course, is how to send the file in the face of all those reasons you can't.

The most common approach is to keep beating your head against the email wall.

You can try to address each of the problems I've enumerated above:

  • send from a domain that doesn't carry such a negative prejudice
  • send the attachments in smaller pieces (as you have)
  • make sure your recipients do everything they can on their end to ensure that your email is as trusted as it can be
  • hope

You might get lucky, and things might improve.

The real solution is not to use email as a way to transport large files. I'll look into the alternatives in a follow-up article: How do I send someone a large file?

Article C3545 - October 26, 2008 « »

Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

Not what you needed?


October 28, 2008 9:21 AM

I wanted to send an 8meg CD to a person on Hotmail. It was rejected. I had them download Gmail, and it went right through.

October 28, 2008 7:17 PM

Sending files using a file splitter or (in the case of PDF file, breaking them into multiple files is fairly good solution, all else being equal. The downside I have experiences with file splitter programs is that some require the recipient to have the same program to convert the multiple files into one file (sometimes you want the recipient to be able to view the file as one, not as multiple files. I suspect Leo is getting to the next suggestion in his follow up article: Use a free online file storage for the file then share it with as many people as you want. Your mileage may vary with the storage service when it comes to how you share, how many people you can share with at one time and the amount of time it takes to upload the file to the storage service.

October 28, 2008 9:50 PM

Try , I signed up for a free account and it was very easy to send a large file to a friend. Go to

October 28, 2008 10:37 PM

I do a little bit of coding, and if I want to send someone an .exe, it can be a nightmare.
In one case I zipped it, then changed the file extension to .jpg and told them to change it back. Luckily they had the knowledge to do so.

November 25, 2008 11:32 AM

The problem (unchanged, as of this morning) is that of my email correspondents, ONE of them is affected thus: His messages containing an "attached file" are stripped of the file, even though the INBOX lineup of messages still shows the Paper Clip icon. Why does AT&T/Yahoo Mail do this? No other correspondent is so affected.

annie jacobs
May 30, 2011 11:55 AM

In some emails I forward the attachments or even scoll downs are not there when rec'd ???

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