Email Attachment Size Limits

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An email with an attached file is larger than the file. Mail servers limit the size of the whole email, and many company mail servers refuse anything over 10 MB, so a 9 MB PDF can bounce from a server whose limit is 10 MB. This page lists the limits of common mail services, mail server software and email sending services, explains how they’re measured, and shows how to choose the largest file to attach.

We checked each limit against the provider’s own documentation in October 2026. Providers change their limits, and an administrator can set a different one for their organization, so treat these as defaults.

Why an attachment makes an email larger

Email is sent as text, so an attached file is encoded with base64 (RFC 2045): every 3 bytes of the file become 4 characters, in lines of at most 76 characters, each followed by a line break. The file takes about 37% more space in the email, and the email’s text and headers add a little more. Providers quote different figures: Google says encoding adds about 37%, and Microsoft says about 33%, which leaves out the line breaks. DocSpring’s emails use shorter lines, which makes an attached PDF 38% larger.

File	Email, about
5 MB	6.9 MB
7 MB	9.6 MB
10 MB	13.8 MB
18 MB	24.8 MB
25 MB	34.4 MB

To find the largest file that fits under a limit, divide the limit by 1.38. A 10 MB limit fits a file of about 7.2 MB, and a 25 MB limit a file of about 18 MB.

What “MB” means

Mail servers don’t all count a megabyte the same way:

Exchange (Server and Online) and Exim count 1 MB as 1,048,576 bytes, so Exchange’s 10 MB is 10,485,760 bytes.
Postfix’s default limit is 10,240,000 bytes.
Google, Amazon SES, Postmark, SendGrid and Mailgun don’t say.

Count 1 MB as 1,000,000 bytes, the smaller: a file under a limit counted that way is under it however the server counts.

Mail services

Service	Receives	Sends	Source
Gmail	50 MB	25 MB of attachments	Gmail Help, Google Workspace Updates
Google Workspace	50 MB, after encoding	25 MB, before encoding	Receiving limits, sending limits
Google Workspace Enterprise Plus	70 MB, after encoding	Up to 50 MB	Google Workspace Updates
Microsoft 365 (Exchange Online)	36 MB, after encoding	35 MB, after encoding	Exchange Online limits
Outlook.com	Not published	25 MB	Microsoft Support
Yahoo Mail	25 MB	25 MB	Yahoo Help
iCloud Mail	20 MB	20 MB	Apple Support
Proton Mail	50 MB	25 MB	Proton Support
Zoho Mail	40 MB	20 to 40 MB, by plan	Zoho Mail limits, plans
Fastmail	70 MB	70 MB	Fastmail Help
GMX FreeMail	20 MB	20 MB	GMX
Yandex Mail	30 MB	25 MB	Yandex Support

Microsoft 365 administrators can set limits from 1 MB to 150 MB, and Google Workspace Enterprise Plus administrators can let people send from 25 MB to 50 MB. Zoho Mail administrators can set 1 MB to 40 MB. Yahoo, Proton and GMX give their limits as attachment sizes, and don’t say whether they count the encoded email.

Mail server software

A company that runs its own mail server has the server’s default limit unless an administrator changes it:

Server	Default limit	Source
Exchange Server 2016, 2019 and Subscription Edition	10 MB, for email it receives from outside too, after encoding	Message size limits in Exchange Server
Postfix	10,240,000 bytes (`message_size_limit`)	postconf(5)
Exim	50 MB, or 52,428,800 bytes (`message_size_limit`)	Exim specification
Sendmail	No limit (`MaxMessageSize`)	Sendmail cf/README

Exchange Server checks email from outside the organization against the organization’s send limit, which is also 10 MB by default. Email security gateways in front of a mail server, such as Mimecast, Proofpoint or Barracuda, have their own limits, and so does every server an email passes through on its way. The smallest limit decides.

Email sending services

Service	Largest email	Source
Amazon SES (API v2 and SMTP)	40 MB, after encoding	Amazon SES quotas
Amazon SES (API v1)	10 MB, after encoding	Amazon SES quotas
Postmark	10 MB, after encoding	Postmark Support
SendGrid	Under 30 MB	SendGrid limitations
Mailgun	25 MB	Mailgun documentation

Finding a mail server’s limit

A mail server announces its limit to the servers that connect to it, with the SMTP SIZE extension (RFC 1870). Find the domain’s mail server, connect to it on port 25 and send EHLO: the SIZE line gives the limit in bytes, where 0 means it has no fixed limit.

$ dig +short MX example.com
10 mail.example.com.
$ telnet mail.example.com 25
220 mail.example.com ESMTP
EHLO test.example.org
250-mail.example.com
250-SIZE 10240000
250 8BITMIME
QUIT

Many internet providers block outgoing connections to port 25, so you may need to run this from a server. The limit a server announces is its own: a gateway in front of it, or the mailbox behind it, can have a lower one. A server that refuses an email for its size replies with code 552, and the sender gets a bounce that says the message is too large or exceeds a fixed size limit.

Choosing the largest file to attach

When you don’t know which mail services your recipients use, keep attachments under 7 MB. The email stays under 10 MB, the most common company limit. Send larger files as a download link.
When every recipient uses Microsoft 365, Gmail or Google Workspace with the default limits, files up to 25 MB arrive: the email is about 34.4 MB.
For recipients on iCloud Mail or GMX FreeMail, keep files under about 14 MB, and on Yahoo Mail under about 18 MB.

Email actions in DocSpring

DocSpring’s email actions email each submission’s PDF to the people who filled in your form or to your team. When an action attaches the PDF, a PDF over its attachment size limit goes as a download link instead. The limit is 7 MB unless you change it, and you can raise it up to 25 MB when everyone the action emails accepts larger emails. See Attachment size limit.