Large email attachments failing to send in Outlook

Outlook often refuses to send large email attachments because it enforces strict size limits on messages and files. Most personal Outlook accounts cap attachments at 20 MB, while business accounts usually top out at 10 MB. If your file is bigger than that, it just won’t go. These limits help keep servers from getting bogged down, but let’s be honest, they can be a real headache when you’re trying to send important documents, images, or videos.

These restrictions can really throw a wrench in your workflow, especially if you’re up against a deadline or sharing big project files. Still, Outlook offers a few workarounds, and with a little know-how, you can send large files without running into endless errors.

At NetTech, we help businesses share large files efficiently and securely, so Outlook doesn’t slow them down. Need advice that fits your setup? Reach out to NetTech Consultants – IT Support and Managed IT Services in Jacksonville.

Understanding Attachment Size Limits in Outlook

When email attachments won’t send in Microsoft Outlook, file size restrictions are usually the culprit. These limits change depending on whether you’re using Outlook.com, the desktop app, or Microsoft Exchange Server. Encoding during transmission can also affect the file size.

Default Attachment Size Restrictions

Outlook sets limits to keep things running smoothly and to play nice with mail servers. Most internet email accounts like Outlook.com or Gmail max out at about 20 MB per message. The Outlook desktop app usually allows up to 25 MB per message, but that includes everything—the attachment and whatever you’ve typed in the body.

You can’t tweak these limits in Outlook’s normal settings. If you try to attach a file that’s too big, Outlook pops up an error and blocks the message. That’s Outlook trying to save you from failed deliveries and overloaded mail servers.

To get around these limits, we usually suggest cloud storage services like OneDrive or SharePoint. Upload your file, send a link, and your recipient can grab it without running into size issues. File compression tools can also help shrink attachments enough to fit under Outlook’s limits.

Microsoft Exchange Server Limitations

If your organization uses Microsoft Exchange Server, you’ll probably see different restrictions. By default, Exchange sets the max message size at 10 MB—that’s attachments plus message text. This is often lower than what you get with a personal Outlook account.

Exchange admins can change these limits through the Exchange Management Console or PowerShell. They can bump up the max send and receive sizes for mailboxes, connectors, and transport rules. This flexibility lets IT teams match limits to business needs.

Still, cranking up the limit too much can slow things down. Big attachments chew up bandwidth, clog mail queues, and eat up storage. We usually recommend a balance: enough flexibility for users, but not so much that the system struggles. For really big files, cloud-based sharing just works better.

Impact of Encoding on File Size

Even if your file looks small enough, encoding can push it over the edge. Email attachments get converted into MIME format, which can make them 30 to 40 percent bigger. So, that 15 MB video? It might balloon to over 20 MB once encoded.

This catches a lot of people off guard. They think their file will fit, but Outlook or Exchange rejects it anyway.

To avoid surprises, keep attachments well below the max size. If the limit is 20 MB, aim for 10 to 12 MB. For anything bigger, we guide clients to secure file sharing platforms that skip email size limits altogether.

Troubleshooting Large Email Attachment Failures

When Microsoft Outlook refuses to send your attachment, it’s usually about file size limits, blocked file types, or outdated software. Figuring out the exact cause lets us fix it faster.

Identifying Error Messages and Causes

Start by checking the error message Outlook gives you. You might see “Attachment size exceeds the allowable limit” or a warning about unsafe file types. These messages aren’t random—they’re tied to Outlook’s rules or mail server settings.

Most internet email accounts, like Outlook.com and Gmail, max out at 20 MB. Exchange and Microsoft 365 accounts might allow more, but admins often set stricter limits. If your file is too big, Outlook just won’t attach or send it.

Sometimes, if your connection is slow or you’re adding lots of attachments, the mail server times out while uploading the file. Noting the exact error helps us figure out if it’s a size problem, a connection issue, or a policy block.

Checking File Types and Formats

Outlook blocks certain file types for security. Executables, scripts, and some compressed files get flagged as unsafe. Even if the file’s harmless, Outlook won’t send it unless you take extra steps.

A good workaround is to compress the file into a .zip folder. That often gets around file type blocks and shrinks the file a bit. If it’s still too big, split it into smaller archives to keep each piece under the limit.

Message format matters too. In HTML, attachments show up in the message body. In Plain Text, they appear below the subject. This doesn’t stop sending, but it can throw people off if they think the attachment is missing. Double-checking the format can save you some confusion.

Outlook Update and Compatibility Issues

Old versions of Outlook can cause attachment failures, especially with cloud services like OneDrive or SharePoint. Older builds might not handle big uploads well or might be missing bug fixes.

Check for the latest Outlook update through Microsoft 365 or Windows Update. Keeping Outlook up to date helps with file handling and security.

Sometimes, add-ins mess with attachments. If updating doesn’t help, try running Outlook in Safe Mode. If things work in Safe Mode, an add-in is likely the culprit. Disabling or removing it usually brings things back to normal.

Make sure your default apps for opening attachments, like PDF viewers, are set up right. If defaults are off, you might not be able to open files even after they’re sent.

Effective Solutions for Sending Large Files

Outlook’s attachment limits can block big files, but there are ways around it. We suggest shrinking files or using a secure cloud platform so your files get where they’re supposed to go.

Compress Files Before Attaching

Compressing files before attaching is one of the easiest tricks. Creating a .zip file reduces the size, and often, that’s enough to get under Outlook’s limit.

Both Windows and macOS have built-in tools for this. On Windows, right-click the file or folder and select Send to > Compressed (zipped) folder. On macOS, just pick Compress from the menu.

Compression works best for documents, spreadsheets, and text files. Media files like images or videos usually don’t shrink much, but you can still group several into one archive for easier sending.

If your zipped file is still too big, it’s probably time to try a cloud option.

Using OneDrive to Share Large Files

If you’re using Microsoft 365, OneDrive is a great way to share big files. Instead of attaching the file, upload it to OneDrive and insert a sharing link in your email. This skips size restrictions and keeps files easy to get to.

Just upload the file to OneDrive (desktop app or browser), right-click, and choose Share. Outlook works with OneDrive, so you can add links straight from your message window.

Recipients can view or download files without extra software. You can set permissions like view only or edit. For sensitive stuff, set expiration dates on links or require sign-in.

This approach works especially well for big videos, project folders, or ongoing collaborations.

Sharing via Google Drive Links

If OneDrive isn’t an option, Google Drive is another solid choice. Upload your file to Google Drive and share a link instead of the file itself.

It’s simple: upload, right-click, select Get link, set permissions, then paste the link into your Outlook message.

Google Drive gives you 15 GB of free storage, which is handy for occasional big transfers. For business, Google Workspace offers more storage and admin controls.

Be careful with sharing options. Anyone with the link can view is quick, but for sensitive info, limit access to specific people. That way, you keep files secure.

Advanced Settings and Administrative Options

If regular file sharing doesn’t cut it, you might need to dig into Outlook’s deeper settings. Adjusting client and admin policies in Microsoft Exchange Server can solve many attachment headaches for businesses.

Modifying Attachment Limits in Outlook

Outlook enforces its own attachment size limit, usually around 20 MB for most accounts. Even if your mail server allows bigger messages, Outlook itself might block them. To match Outlook’s limit to your server’s, you can change it in the Windows Registry.

Here’s the usual registry path:

HKEY_CURRENT_USER\Software\Microsoft\Office\<version>\Outlook\Preferences

Add or update the MaximumAttachmentSize value. Enter the number in kilobytes (KB), so 153600 lets you attach up to 150 MB. Setting it to 0 removes Outlook’s limit, but remember, your server will still enforce its own.

We recommend IT administrators handle these changes centrally using Group Policy for consistency. That way, users don’t accidentally set the wrong values, and Outlook stays in sync with Exchange limits. Keeping both sides aligned reduces annoying send errors.

Configuring Exchange Server Policies

Microsoft Exchange Server sets its own message size rules that actually override whatever you specify in Outlook. In Exchange Online, the default max is 35 MB for sending and 36 MB for receiving, but admins can bump this up to 150 MB if they really need to.

Inside the Exchange admin center, here’s what we usually do:

1. Go to Recipients > Mailboxes
2. Pick a user or a bunch of users
3. Change the Message size restrictions in the Mail flow settings

When you’re dealing with a big environment, PowerShell just makes life easier. For example:

Set-Mailbox -Identity "user@domain.com" -MaxSendSize 150MB -MaxReceiveSize 150MB

You can also set limits for every mailbox or tweak defaults for new accounts with mailbox plans. These changes usually kick in after a few minutes.

Managing these settings at the server level gives you a lot more control over how attachments behave. You’ll probably see fewer failed deliveries because you’ve eliminated mismatched limits across the board.