How is a large file sent by email?
A large file is sent by email not as an attachment but as a secure download link: the file is uploaded to storage, only its link is added to the email, and the recipient downloads it with one click. In this guide, we explain where the email attachment size limit comes from, how to send files that exceed the limit, and how files up to 250 MB are transferred in SmartMail.
Why is there an email attachment size limit?
Email, when first designed, was a system intended for short text messages. When a file is attached to an email, it is carried by being converted into text (with base64 encoding), and this conversion increases the file's size by about one third. A 20 MB presentation becomes an email of nearly 27 MB in transit.
An email is carried from the sender's server to the recipient's server through several stops, and each stop has its own size limit. Most common mail servers accept emails of around 25 MB and reject larger ones. The problem is that the sender does not see the rejection immediately: the email appears to have left the outbox, and a few minutes later an "undeliverable" notification arrives. Sometimes the recipient's server silently drops the email and the recipient never sees the attachment.
Therefore the limit is not arbitrary; when exceeded, sending the file as an attachment is not reliable.
What are the ways to send a large file?
Reducing the file size. Presentations and PDFs are often larger than necessary. When high-resolution images are compressed or a PDF is saved at "screen" quality, the size drops noticeably. This method is the simplest, but it does not suit every file; technical drawings, videos and print-ready files cannot be reduced.
Splitting the file into parts. It is possible to split an archive into several parts and send them in separate emails, but it is cumbersome for the recipient and the file will not open if one part is missing.
Sending a download link. The file is uploaded to a storage area and only its link is added to the email. The email itself stays small, and the recipient downloads the file whenever they want. This is the most reliable method for large files.
What should be considered when sending a download link?
The link should be unguessable. A sequential or short link can be guessed by others and lead to the file being downloaded. A secure link contains a long, random key.
It should not create extra hassle for the recipient. A customer or supplier is often not a user of the system you use. Asking them to create an account or sign in to download the file slows things down.
The link should be saved with the email. It should be visible on the email itself which email the file was sent with, to whom and when. Links created in a separate file-sharing tool leave the question "who did we send this file to?" unanswered months later.
Dangerous file types should be blocked. Executable files (such as .exe, .bat, .js, .iso) should not be sent in business correspondence, either as attachments or as links. These files are considered suspicious by the recipient's security system and can damage your organization's reputation.
The file name should be clear. A clear name such as "2026-09_Quote_ABC-Makine.pdf" instead of "final_final_v3.pdf" lets the recipient understand what the file is without opening it.
Which files usually exceed the limit?
In some sectors, sending large files is everyday work:
- Production and engineering: CAD drawings, technical specifications, 3D models.
- Architecture and construction: project drawings, render images.
- Marketing and media: promotional videos, print-ready catalogue files.
- Legal and accounting: scanned file archives, document sets hundreds of pages long.
- Training and consulting: recorded presentations and training videos.
For these teams, keeping large file sending within the email flow is safer and more traceable than the habit of using a separate tool.
What should be considered on the recipient's side?
The success of sending a large file is measured by the recipient receiving it without problems. Even if everything is done correctly on the sender's side, several obstacles may arise on the recipient's side.
Trust in the link. Corporate security systems are cautious about links they do not recognise; employees are also trained not to click links they do not know. That is why it is important to write a clear sentence next to the link introducing the file: "The 2026 catalogue print file (180 MB) is included as a download link." A link to an expected file is much more likely to be opened than an unexpected link.
Download time. Large files take a long time to download on slow connections. If the recipient will need the file immediately, sending should not be left until the last minute.
The file can be opened. Make sure the recipient has the software to open the file. If a common format (such as PDF) can be sent instead of a file from a specialist design program, prefer that.
Confirmation. For critical files, ask the recipient to confirm with a short reply that they have downloaded the file. In time-sensitive work such as quotes and tenders, this confirmation prevents a dispute that could arise later.
How does SmartMail send large files?
In SmartMail, files up to 25 MB are sent as normal attachments. Files exceeding 25 MB, up to 250 MB, are sent as a secure download link instead of an attachment. The user attaches the file to the email as usual; if the file is large, it appears in the composition screen as a link.
The download link carries an unpredictable key generated from 32 random bytes. The recipient does not need to create an account or sign in to download the file; clicking the link is enough. The file is associated with the email sent, so you can see in the outbox which file went with which email.
Executable and risky file types are blocked both as attachments and as download links. The list covers types that are not needed in business correspondence and are misused, such as .exe, .bat, .cmd, .scr, .msi, .js, .vbs, .ps1, .jar, .lnk, .hta, .iso, .img and .vhd. Concealment tricks such as dots and spaces added to the end of the file name are also caught.
Sent files take up space in the organization's storage; storage usage per mailbox is monitored and the "Free up space" screen shows what is taking up space.
Sending a large file step by step
- Check the file size. If it is under 25 MB, send it as a normal attachment.
- Reduce the size if needed. Try saving PDFs and presentations at screen quality.
- Attach to the email. Files over 25 MB, up to 250 MB, are attached as a link.
- Introduce the file in the email body. Write in a sentence or two what the file is and why it is being sent; the recipient will click the link with confidence.
- Check in the preview. Before sending, make sure the link appears in the email.