What is the maximum size of an email?
Emails up to 100 MB in size can be sent and received via our mailboxes. In practice, this means that an attachment should not exceed 80 MB.
However, please consider before sending that not all recipient mailboxes support such large emails and there may be reception errors. If necessary, for very large files, you should consider sharing the data via one of our Nextcloud products. Here, you can, for example, share files via a download link even for unregistered users.
Why is the attachment limit lower than the email limit
Email systems measure size based on the entire message, not just the raw attachment file. When you attach a file, most mail systems encode it (typically using Base64), which increases its size. As a result, an attachment that is 80 MB on disk can become significantly larger once it is encoded and wrapped inside the email message along with headers and body content. This is why an email provider may state a 100 MB maximum email size but recommend a lower practical attachment size.
In addition to encoding overhead, the email also includes headers, formatting, and sometimes inline elements (such as embedded images in the message body). These components consume part of the total allowed size. If you try to attach a file close to the maximum, the “on-the-wire” message may exceed the limit even though the original file appears to fit.
The practical takeaway is simple: treat the published maximum as the ceiling for the whole message, and use the recommended attachment guideline as the safe operational limit to avoid rejections.
Best practices for sending large files safely
Large attachments are among the most common causes of mail delivery failures because the sender and recipient may have different size limits. Even if your mailbox allows large messages, the recipient’s mail provider may reject them, quarantine them, or delay them. For this reason, attaching huge files is rarely the most reliable option.
A good rule of thumb:
Attach files when they are small enough to be universally accepted and when the recipient explicitly expects an attachment.
Share a link when the file is large, sensitive, or needs version control.
Link-based sharing is often better for business workflows because it supports access control, expiry, and re-download without resending. It also avoids mailbox bloat and reduces the chance of delivery failure. If you regularly send files near the upper attachment range, standardize on file sharing for reliability. This is also more professional: recipients can download at their convenience, and you avoid repeated “message rejected” back-and-forth.
How to reduce file size before sending
If your file is slightly too large to send, you can often reduce it enough without compromising usability. Start with the simplest reductions first; these typically save the most time.
Quick options:
Compress the file into a ZIP archive. This works best for documents and datasets; it is less effective for already-compressed formats (JPEG, MP4, PDF with images).
Reduce the image size or quality before attaching (export at a lower resolution or higher compression). For many use cases, a 1920px width is sufficient and reduces the size drastically.
Export PDFs with compression or “optimized for web” settings. Many PDF tools can significantly reduce image-heavy documents.
Split archives into multiple smaller files if the recipient accepts multiple emails (for example, two 40 MB attachments instead of one 80 MB attachment).
Remove unnecessary content such as duplicate versions, embedded media, or unused high-resolution assets.
If the file still remains large after basic optimization, switching to link-based sharing is usually faster and more reliable than continuing to compress.
What rejection messages typically mean
When a message fails due to size constraints, the mail system usually returns a bounce or error message indicating why delivery was rejected. Understanding the category of error helps you quickly choose the correct fix.
Common outcomes and recommended next steps:
“Message size exceeds fixed maximum” / “Message too large” This usually means your message exceeded the sender-side or relay limit. Reduce attachment size or use file sharing.
Recipient rejects large messages Even if your provider allows it, the recipient’s provider may enforce a smaller limit. In this case, resending the same attachment will not help. Use a link instead.
Temporary rejection due to policy Some providers temporarily defer large messages. If time-sensitive, do not wait—use a link-based method immediately.
The best practice is to avoid retrying the same attachment when the error indicates a hard size limit. Switch to a shared link, confirm the recipient can access it, and keep email as the notification channel rather than the transport channel for large files.