What counts as an email attachment
An email attachment is any file you send alongside the message body — a PDF, a photo, a spreadsheet, a presentation, a ZIP archive. The file is encoded into the message itself, which is why every provider caps how large it can be: the attachment does not travel as a separate download, it travels inside the email.
That single fact explains almost every attachment problem people run into. Size limits exist because the file is part of the message. Encoding inflates the file by roughly a third on the way out, so a file that looks like it fits often does not. And because both the sending and the receiving server enforce their own cap, the smaller of the two always wins.
This page covers the whole subject: what the limits actually are, how to attach files on each platform, what to do when a file is too big, and how to keep attachments safe. Each section links to a detailed guide if you need the step-by-step.
Attachment size limits by provider
These are the per-message caps for the major providers. They apply to the total message — all attachments plus the body and headers combined, after encoding.
| Provider | Send limit | Receive limit | Notes |
|---|---|---|---|
| Gmail / Google Workspace | 25 MB | 50 MB | Files over the cap are offered as a Google Drive link instead |
| Outlook.com | 20 MB | — | OneDrive link offered for larger files |
| Microsoft 365 / Exchange | Admin-set, commonly 25–35 MB | Admin-set | Tenant policy overrides the default |
| Yahoo Mail | 25 MB | — | Requires an app password for third-party clients |
| iCloud Mail | 20 MB | — | Mail Drop handles larger files separately |
Two things trip people up here. First, the receive limit matters as much as the send limit — a 24 MB file leaves Gmail fine and still bounces if the recipient's server caps at 20 MB. Second, the number you see in your file manager is not the number the server measures: base64 encoding adds roughly 33%, so a 20 MB file arrives at the server as about 27 MB.
For the full breakdown per provider, see email attachment size limits and maximum email attachment size, and how big an email attachment can be.
How to attach a file
The mechanics are the same everywhere — find the paperclip, pick the file, wait for the upload to finish before you hit send. What differs is where the control lives and what happens when the file is too big.
In a desktop client
Compose a message, click the paperclip in the toolbar, and select the file. You can also drag a file straight onto the compose window, which is usually faster. The attachment is uploaded as you compose, so on a slow connection the send button may sit unresponsive for a moment while the transfer completes — that is expected, not a failure.
In webmail
Gmail, Outlook.com and Yahoo all behave the same way: paperclip, choose file, wait for the progress bar. Gmail and Outlook.com will intercept an oversized file and offer to convert it to a cloud link automatically.
On mobile
Attachment handling on phones runs through the share sheet rather than the mail app itself. It is often easier to open the file first — in Files, Photos or Drive — and share it to your mail app than to start from a blank email and go looking for the file.
Step-by-step walkthroughs: how to attach a file to an email, how to send an attachment in an email, attaching files the right way, and how to add an attachment in Outlook.
Two adjacent tasks have their own mechanics: writing the message that carries the file, and forwarding an email as an attachment — which sends the original message itself as a file rather than re-attaching what was inside it.
When the file is too large
There are only three real options, and they are not equally good.
- Compress it. Works well for documents and uncompressed images. Does almost nothing for video, MP3s or already-compressed archives — those formats are compressed already, so zipping saves a few percent at best.
- Send a cloud link instead. Upload the file to storage and paste a share link into the message. This is the option that actually scales: no size cap, the recipient downloads at their own pace, and you can revoke access later. It is what Gmail and Outlook.com push you toward automatically.
- Split the file. A last resort. Multi-part archives are fragile, require the recipient to have the right tool, and fail if any part goes missing.
For anything above about 20 MB the answer is almost always a cloud link. Detailed guides: sending large video files, sending large files through email, and how to send a large attachment and avoid errors.
Using cloud storage without leaving your inbox
The friction in the cloud-link approach is never the upload — it is the context switch. Open a browser tab, sign in to storage, find the folder, upload, wait, generate a share link, copy it, switch back to your mail, paste. Seven steps for one attachment, and every one of them is a chance to lose your place.
A desktop client can collapse that. Mailbird runs Google Drive, Dropbox and OneDrive as apps inside the client itself, in a side panel next to your inbox. You upload the file from that panel, copy the share link, and paste it into the message you already have open — the whole sequence happens without leaving the application or opening a browser.
For Dropbox and Degoo it is tighter still: both integrate directly with the compose window, so you can pick a file from a chooser inside the message you are writing and have a share link created for it there.
The practical gain is not that any single step disappears — it is that the whole task stays in one window. If you send large files regularly, that is the difference between a routine action and an interruption.
On whether to attach or link in the first place, see PDF attachment vs link.
Attachment safety
Attachments remain one of the most common malware delivery routes, and the reason is behavioural rather than technical: a file that arrives from a familiar name gets opened.
The rules worth internalising are short. Treat any unexpected attachment as suspect even when the sender looks legitimate — display names are trivially spoofed. Be wary of file types that can execute: .exe, .scr, .js, .vbs, and macro-enabled Office files (.docm, .xlsm). Watch for double extensions like invoice.pdf.exe, which rely on your system hiding the real one. And turn off automatic image loading, which quietly confirms to a sender that your address is live.
Client-side habits supplement provider filtering rather than replacing it — the provider's scanner catches the bulk, and judgement catches what gets through. More detail: when it is safe to open an unexpected attachment.
Attachments in Mailbird
Mailbird is a desktop email client for Windows and Mac. Attachments work the way they do in any client — paperclip or drag-and-drop, with the same provider size caps applying, because those caps belong to the provider and no client can raise them.
What changes is the surrounding workflow. Every account you connect lives in one unified inbox, so a file that needs forwarding from a work account to a personal one does not involve switching windows. Cloud storage sits in the same window as your mail. And attachment search runs across all connected accounts at once, which matters more than it sounds when you are looking for a document you know you were sent but cannot remember where.
Email attachment FAQ
What is the maximum size for an email attachment?
Most providers cap a message at 20–25 MB: Gmail and Yahoo at 25 MB, Outlook.com and iCloud at 20 MB, and Microsoft 365 at whatever the tenant administrator sets, commonly 25–35 MB. The cap applies to the entire message after encoding, not to each file, and the recipient's receive limit applies too — so the smaller of the two limits is the one that decides whether your message arrives.
Why did my attachment fail to send when it was under the limit?
Almost always because of encoding overhead. Attachments are base64-encoded for transport, which increases size by roughly 33%, so a 20 MB file is measured by the server at about 27 MB. The other common cause is the recipient's server having a lower receive limit than your send limit.
Is it better to attach a file or send a link?
Attach when the file is small, final, and needs to sit in the recipient's mailbox as a record — invoices, signed contracts, anything they will search for later. Send a link when the file is large, when it may be revised, or when several people need it. Links also let you revoke access, which an attachment never does once sent.
Can I upload to cloud storage without leaving my email client?
Yes. Mailbird runs Google Drive, Dropbox and OneDrive as apps in a panel beside your inbox, so you can upload a file and copy its share link into the message you are composing without opening a browser. Dropbox and Degoo go further and integrate with the compose window directly, letting you choose a file and generate a link from inside the message itself.
Does zipping a file make it small enough to email?
It depends entirely on the file type. Documents, spreadsheets and uncompressed images can shrink substantially. Video, audio and existing archives are already compressed, so zipping them typically saves single-digit percentages — not enough to bring a large file under a 25 MB cap. For those, use a cloud link.
Which attachment types are most dangerous?
Anything that can execute code: .exe, .scr, .bat, .js, .vbs, and macro-enabled Office documents such as .docm and .xlsm. Also watch for double extensions like invoice.pdf.exe, which exploit the fact that many systems hide the final extension by default. Treat any unexpected attachment as suspect regardless of who appears to have sent it.
