Email Size Limits: Outlook, Gmail and Yahoo Attachments

Email size limits by provider

Read each row as the ceiling for one message: the body, inline images and every attachment together, unless the row says attachments only.

The Outlook rows are the ones people mix up. “Outlook” is an app, and the limit belongs to the account inside it. The same classic Outlook window can hold an Outlook.com account (25 MB), a work mailbox on Microsoft 365 (35 MB unless your admin changed it) and a Gmail account added over IMAP, where Outlook itself refuses anything over 20 MB of attachments before Gmail is even asked. An on-premises Exchange Server starts at 10 MB. The new Outlook and Outlook on the web have no limit of their own; they apply the account's.

“Outlook email limit” sometimes means a sending cap instead: a Microsoft 365 mailbox can send to 10,000 recipients a day and 30 messages a minute, Outlook.com to 5,000 recipients a day and 500 per message.

Why a 20 MB file fails under a 25 MB limit

Email is text. An attachment travels as base64, which writes every 3 bytes of the file as 4 characters and breaks the result into 76-character lines (RFC 2045). The file grows by about 37% on the way out. Google quotes that figure for its receiving limits, and Microsoft budgets 33% for mail that leaves its datacenters, which is why Microsoft 365's 150 MB ceiling becomes 112 MB for mail to other providers.

// Bytes on the wire for a file sent as an attachment.
// Base64 turns every 3 bytes into 4 characters (RFC 2045)
// and adds a CRLF after every 76 characters.
function encodedSize(bytes: number): number {
  const base64 = Math.ceil(bytes / 3) * 4;
  return base64 + Math.ceil(base64 / 76) * 2;
}

encodedSize(20 * 1024 * 1024); // 28,697,872: a 20 MB file is ~27.4 MB sent
encodedSize(18 * 1024 * 1024); // 25,828,084: ~24.6 MB, just under a 25 MB cap

Where a limit is measured on the encoded message, as Google says its receiving limit is, 25 MB holds about 18 MB of files. Keep attachments for an outside recipient under that, and the message passes almost any server that accepts 25 MB.

The smallest limit on the path wins

A message passes at least three checks: your mail app, your provider's outgoing server and the recipient's incoming server. Gmail lets you attach 25 MB of files; encoded, that is about 34 MB, so a recipient whose server caps messages at 25 MB never gets it, and the bounce lands in your inbox.

A server announces its ceiling in its reply to EHLO, the SIZE extension of RFC 1870, in bytes. You can read it yourself (the full walk-through is in how to test an SMTP server):

$ openssl s_client -starttls smtp -connect smtp.example.com:587 -quiet
220 smtp.example.com ESMTP
EHLO me
250-smtp.example.com
250-SIZE 36700160
250 ...

Here, 36,700,160 bytes is 35 MB. When a message is too big, the bounce says so with a 552 reply and the enhanced code 5.3.4 (“message too big for system”) or 5.2.3 (over a per-mailbox limit). Both are permanent: resending fails the same way. How to read bounce codes covers the rest.

How to send a file that is too big

• Send a link. Gmail does it for you above 25 MB and attaches a Google Drive link. Outlook.com and Microsoft 365 attach OneDrive files up to 2 GB as links, which don't count toward the email size limit. Check the sharing setting, or the recipient gets a request-access page.
• Zip it, if it is a document, spreadsheet or folder of text. JPEG, MP4 and most PDFs are already compressed and barely shrink.
• Shrink the images. Photos straight off a phone run to several MB each, at a resolution no email needs.
• Split it across several emails, as a last resort.

How to reduce email size in Outlook

In classic Outlook, attach the images, then choose File > Info and tick Resize large images when I send this message under Image Attachments. For pictures placed in the body, select them and use Picture Format > Compress Pictures.

The new Outlook and Outlook on the web can change how big a picture looks, but Microsoft says they cannot reduce its file size. Resize the image before you insert it, or send a OneDrive link.

For IMAP and POP accounts, classic Outlook's 20 MB check can be raised with the MaximumAttachmentSize registry value (in KB; see the FAQ below). That only removes Outlook's own check; the servers still apply theirs.

The body has its own limit: Gmail clips at 102 KB

For HTML email the limit that matters is smaller: Gmail accepts a 25 MB message but cuts its HTML body after 102 KB and hides the rest, unsubscribe link included, behind “View entire message”. If you send HTML email, that is the size to watch: host your images instead of embedding them as base64, which counts toward both limits, and measure the HTML before you send. The details are in why Gmail clips emails over 102 KB, and the size checker measures yours.