There is no limit set by email standards outlined in RFC (Request For Comments) memorandums, however, due to technical issues that arise from sending large files, most webmail email services set a limit of 1mb-25mb for file attachments. In 2016, AOL, Yahoo, Gmail and Outlook.com (formally Hotmail) had a single file attachment limit of 25MB. Additionally, if more than one file attachment is added to the email, they cannot total more than 25MB. Mail.com currently has the highest total attachment limit of 50MB, but it is limited to 10MB per file.
It is often asked why, in 2017, is there a limit imposed on webmail attachments? The problem lies with the Internet protocols designed for email (SMTP, POP, IMAP): they were designed in the 1980s, and they were designed to send text-only email messages. The ability to add attachments to email was an after-thought: the Multipurpose Internet Mail Extensions (MIME) was released in the 1990s, it was designed for use with SMTP, and it was the first Internet standard that provided support for non-text attachments: video, audio, images, programs etc.
Due to email protocols being designed for text-only email, no file compression mechanism exists within the protocols, nor any mechanism to resume a broken download. The core email protocol is SMTP: which uses a store-and-forward method to transport email messages, plus their attachments, from MTA (mail transfers agent) to MTA, until it reaches its destination. Therefore, each MTA will need to copy and store the attachment file: obviously if this file was 1000MB, the pressure, plus cost, placed upon the operators of MTA's would increase exponentially.
While, the traditional method of sending webmail attachments is limited to
50MB, Yahoo Mail has recently introduced a new method for webmail attachments:
that uses cloud computing to send the attachment: for files attached via the
DropBox cloud storage, there is no size limit applied to the attachment. For
the standard method of attaching files, Yahoo Mail still has a 25MB limit.
Outlook.com has followed suit by providing a 300MB limit for files attached
using the Skydrive cloud storage service. Other webmail services should soon
follow suit and offer cloud computing attachments: the problem presently,
is that you need an account at a specific cloud storage services, hopefully
you'll be able to attach files from any of the main cloud computing services:
Sync.com, pCloud, SpiderOak, IDrive, MEGA, OneDrive, Google Drive, Box, Egnyte,
Amazon Drive or Final Thoughts.
So, what happens when you attempt to exceed the size limit for a webmail attachment?
Typically, an error will be returned (example shown above: mail.com), before the email is sent.