Editing Email attachment (section)

==Size limits==
Email standards such as [[MIME]] do not specify any file size limits, but in practice, email users will find that they cannot successfully send very large files across the Internet.

This is because of a number of potential limits: 
* Mail systems often arbitrarily limit the size their users are allowed to submit.<ref>[http://exchangepedia.com/2007/09/exchange-server-2007-setting-message-size-limits.html ''"Setting Message Size Limits in Exchange 2010 and Exchange 2007"''];</ref> 
* A message will often pass through several [[mail transfer agent]]s to reach the recipient. Each of these has to store the message before forwarding it on, and may therefore also impose size limits. 
* The recipient mail system may reject incoming emails with attachments over a certain size.

The result is that while large attachments may succeed internally within a company or organization, they may not when sending across the Internet.

As an example, when [[Google]]'s [[Gmail]] service increased its arbitrary limit to 25MB it warned that: "''you may not be able to send larger attachments to contacts who use other email services with smaller attachment limits''".<ref>[http://www.geek.com/articles/news/google-updates-file-size-limits-for-gmail-and-youtube-20090629/#ixzz0oIzFY0Q8 ''"Google updates file size limits for Gmail and YouTube"'', geek.com] {{Webarchive|url=https://web.archive.org/web/20111219141547/http://www.geek.com/articles/news/google-updates-file-size-limits-for-gmail-and-youtube-20090629#ixzz0oIzFY0Q8 |date=2011-12-19 }}.</ref><ref>[http://mail.google.com/support/bin/answer.py?answer=8770&topic=1517 ''"Maximum attachment size"'', mail.google,com].</ref> 

Also note that all these size limits are based, not on the original file size, but the [[MIME]]-encoded copy. The common [[Base64]] encoding adds about 37% to the original file size, meaning that an original 20MB file could exceed a 25MB file attachment limit.<ref>{{cite web|url=http://luxsci.com/blog/raw-vs-encoded-email-message-size-whats-the-difference.html|title=Raw vs. Encoded Email Message Size — What's the Difference?}}</ref>
A 10MB email size limit would require that the size of the attachment files is actually limited to about 7MB.

Users should be cautious with certain file formats when received as email attachments, such as [[ZIP (file format)|.zip]] and [[.tgz]] files, because they can contain harmful viruses and potential software. .iso files can also be used to spread malware and [[.exe]] is an executable file that can become active on a computer as soon as it is opened.