There are many reasons emails with attachments fail to be delivered.
Heck, there are many reasons email without attachments might not make it. 🙂
Focusing on just the attachment part of the equation, however, there are a number of potential problems, and I’ll try and touch on a few of them.
We can start by blaming malware.
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Problem #1: Spam filters
Perhaps the single biggest issue is that emailed attachments are the number one vector for malware. The result is that ISPs, mail programs, and mail servers are all extremely suspicious of any and all attachments.
You should be skeptical as well, but you have the advantage of knowing whether or not you can trust the sender and whether or not you’re expecting an attachment. Spam filters don’t have that context.
An attachment is a strike against an email when a recipient’s spam filter analyzes a message. Put another way: if it has an attachment, it’s slightly more likely to be considered spam.
Problem #2: Reputation
Every aspect of an email carries with it a reputation. That includes:
- The From: address.
- The domain (i.e. @hotmail.com) used in the From: address.
- The email service used to send the email.
- The specific email server used to send the email.
The reputation of each of those items, and probably a few more I can’t think of, contributes to an email’s probability of being flagged as spam.
Free email services in particular (Hotmail, Outlook.com, Gmail.com, etc.) have a slightly higher chance of being flagged as spam than, say, email from a business domain using a high-quality email-sending service. The reason is simple: those free services have been abused by spammers for years.
The most important thing to realize is that reputation matters. Any item — from your personal email address to the specific server your email service happens to use — can be considered a strike against you if that item has a bad reputation for having sent spam in the past.
Problem #3: Explicit attachment blockers
For the very same reason mail systems consider attachments risky, many email programs now come pre-configured to block access to some or all attachments, and many email systems — most notably corporate systems — often do the same.
The recipient must somehow indicate which attachments are accepted. Depending on their email program or system, 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.
Problem #4: Size matters
Large files have always been a problem for email, and ISPs regularly disallow emails that exceed a certain size. Attachments are often the reason, since they quickly add heft to any email message.
The problem is that even though “a certain size” might be getting larger over time as the internet and its technologies continue to improve and grow, it also varies from ISP to ISP. If your ISP allows you to send a large file, it doesn’t guarantee 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. Unlike simply copying a file from one machine to another, where only two machines are impacted, an email travels across several servers, and each of those has to have the resources to handle it without adversely impacting other email deliveries.
Solution #1: Stack the deck in your favor
It’s impossible to guarantee that an email — with or without attachments — will be delivered1, but you can stack the deck in your favor.
- To the extent you can, send from email domains and services with a good reputation.
- Send smaller attachments, as you have, by sending multiple attachments in separate emails rather than together.
- Ask your recipients to do what they can on their end to ensure that your email is as trusted as it can be.
You might get lucky, and things might improve.
Solution #2: Don’t send attachments
Honestly, this is the real solution: don’t send files as attachments at all. Instead, use a cloud storage service like Dropbox, OneDrive, Google Drive, or others.
The process is fairly simple:
- Upload your file or files to your cloud storage provider.
- Get a sharing link2 from that provider (usually by right-clicking on the file or folder in that provider’s interface).
- Include that link in the email you send to your intended audience, rather than including the actual file(s).
- Your recipient can then download the file as needed.
Your email will sidestep some of the issues I’ve identified above, will probably be delivered more quickly, and will have a greater probability of making it to your recipient. It’s still not guaranteed, but you’ll have stacked the deck in your favor even further.
1: To be fair, the system is amazingly resilient, and most email is delivered, and even delivered quickly.
2: Generally, this is a public link that anyone can view, but if you and your recipient both use the same cloud storage provider, you can often restrict who is allowed to view the uploaded file.