Mimecast does not have one fixed attachment limit — it enforces whatever maximum message size your company's IT team configured, commonly somewhere between 25 MB and 100 MB. When an email exceeds it, Mimecast rejects or holds the message at the gateway, often after you have already hit send. If large files are part of your job, you have three sanctioned ways through: Mimecast's own Large File Send add-on, a transfer link, or getting the policy changed. Here is how each works.
Why your attachment bounced even though Gmail/Outlook accepted it
Mimecast sits in front of your company's mail servers as a security gateway, inspecting every inbound and outbound message. Delivery is governed by the lowest limit in the chain: your mail client may happily accept a 40 MB attachment, but if the Mimecast policy says 30 MB — or the recipient's gateway says 20 MB — the message bounces with a size-exceeded error. Add the ~33% Base64 encoding overhead on attachments (see our provider limits table) and a "25 MB" policy really fits an ~18 MB file.
Option 1: Mimecast Large File Send (if your company licensed it)
Mimecast's answer to its own limits is the Large File Send add-on: from Outlook (or the web portal), oversized attachments are diverted to Mimecast's cloud and the recipient gets a secure download link — typically supporting files up to around 2 GB, with optional access keys and expiry. If your organization pays for it, this is the IT-blessed route. How to tell: look for a Mimecast ribbon in Outlook, or ask IT whether "Large File Send" is enabled for your account.
Option 2: Send a transfer link instead (works everywhere, today)
A download link weighs a few bytes, so it passes through Mimecast — and every other gateway — untouched. This is also what Large File Send does under the hood; you are simply choosing the service:
- Upload the file at everytransfer.com — no account needed, TLS in transit, AES-256 encryption at rest available.
- Set a password, expiry date, and download limit — the controls compliance teams want to see.
- Email the link normally through your corporate mail. The message sails through the gateway.
- Share the password via a separate channel (Teams, phone) for sensitive material, and watch the download notification confirm receipt.
One professional courtesy: some organizations restrict which external services staff may use for company data. If that is your environment, point IT at the security page — password protection, encryption at rest, expiry, download limits, and audit-friendly download logs — or use whichever transfer tool your policy already approves. The pattern (link instead of attachment) is the same. Our guide to the safest ways to send sensitive documents covers the controls in depth.
Option 3: Ask IT to raise the policy (when it is genuinely too low)
Admins can raise Mimecast message-size policies — globally or per group — and many organizations run higher limits for teams that legitimately exchange big files. If your whole department fights the gateway weekly, a policy request with concrete examples beats a hundred workarounds. Expect a ceiling regardless: email infrastructure everywhere struggles past ~100–150 MB per message, which is why links remain the real answer at scale.
The three routes compared
| Route | Max size | Needs IT involvement? | Recipient experience |
|---|---|---|---|
| Mimecast Large File Send | ~2 GB | Yes — licensed add-on | Secure Mimecast download page |
| Transfer link | GBs free / unlimited paid | No (subject to data policy) | Branded download page, no account |
| Raised gateway policy | ~100–150 MB practical ceiling | Yes — admin change | Normal attachment |
What is the maximum email size Mimecast allows?
Whatever your administrators set — Mimecast itself supports large policy values, and real-world deployments commonly sit between 25 MB and 100 MB per message. There is no universal number; the bounce message you received reflects your organization's (or your recipient's) specific policy.
Why did my email get held instead of bounced?
Mimecast policies can hold oversized or suspicious messages for review rather than rejecting them outright. Held mail sits in a queue until an admin releases it — which can take hours. For anything deadline-driven, a transfer link avoids the queue entirely and gives you a download receipt instead of silence.
"Gateways filter messages, not links. Send the link, keep the audit trail, skip the bounce."
Send a large file past the gateway — free