Best Practices for Email Attachments

Size of Messages and Attachments

Messages should always be kept as short as possible. This is particularly important when sending messages to large mailing lists.

Non-textual files, such as Word documents or spreadsheets, are generally quite large. For instance, a file of 2,000 characters (about 300 words) could become a 20 KB file when saved as a Word document. Attaching files to messages should be avoided as much as possible.

Compress file attachments using freely available software such as WinZip or other common tools.

Whenever possible, files should be saved as HTML (see below) and installed on the Web. The URL can then be included in a short message.

Keep in mind that Internet email is not secure. Internet email should be thought of as no more secure than a postcard.

The mail hubs restrict the total size of each message that goes to or comes from the Internet:

  • The maximum size of messages sent via the university email system is 25 MB (including the body of the message and the attachments). Other departmental SMTP servers may impose their own limits (or none at all).
  • The maximum size of messages cent via the central SMTP servers is 25MB (including the body of the message and the attachments). Other departmental SMTP servers may impose their own limits (or none at all).
  • The University email system does not accept messages larger than 25MB. Other message delivery systems may impose their own limits (or none at all).

You can freely exchange large files using free utilities on the Internet such as:

These files can then be accessed by anyone who has been given their location.

Using a free large file exchange program will allow you to distribute very large files to your recipients by sending them an email that contains a hyperlink to the file. When the recipient clicks the hyperlink, they download the large file from the system's web site. Note that if security of files is a concern, the files should not be sent via Internet mail of any kind because Internet mail is inherently insecure.

Format of Attachments

If you do have to attach a non-textual file to a message (for example, a document that is being sent to a few people for comments), remember to ensure that the recipients have access to software that will be able to read the file. In particular, if you are using a relatively new version of a package, ensure that the people to whom you are sending the file are also using that version. Many application vendors supply web plug-ins to allow recipients to view a document in a browser without needing to purchase the authoring software.

Some free document viewers:



Dangerous Attachments

Use extreme caution when receiving file attachments from an unknown or untrusted source. Even if you use antivirus and anti-malware protection, clicking on unknown attachments can damage your machine.

The university email system blocks the attachement of executable programs (usually *.exe files), files that can't be zipped and zip files within zip files. For the latest information, view Google Gmail—Some file types are blocked.

Files with these extensions are blocked by our inbound SMTP system

To further combat malware/virus attacks, the campus desktop support requested in Spring 2007 that dangerous file attachments be blocked. The following types of files are recognized by the IT industry as dangerous attachments. If someone from an off-campus email system tries to send these files, the file will not arrive in your campus mailbox.

*.ade, *.adp, *.app, *.asa, *.asp, *.asd, *.awk, *.bas, *.bat, *.cdx, *.cer, *.cgi, *.chm, *.cil, *.chm, *.class, *.cmd, *.com, *.cpl, *.crt, *.csh, *.dek, *.dll, *.ds, *.eml, *.emf, *.esh, *.exe, *.ezs, *.fky, *.fxp, *.hlp, *.hta, *.htr, *.htw, *.ida, *.idc, *.idq, *.inf, *.ins, *.inx, *.ipf, *.isp, *.its, *.jar, *.js, *.jse, *.ksh, *.lnk, *.mad, *.maf, *.mag, *.mam, *.maq, *.mar, *.mas, *.mat, *.mau, *.maw, *.mda, *.mdb, *.mde, *.mdt, *.mdw, *.mdz, *.mem, *.mpx, *.msc, *.msi, *.msp, *.mst, *.nws, *.obs, *.ocx, *.ops, *.pcd, *.pif, *.prf, *.prg, *.printer, *.pst, *.pvd, *.pwc, *.pyc, *.pyo, *.pqx, *.reg, *.rgs, *.rox, *.scf, *.scr, *.sct, *.shb, *.shs, *.shtml, *.stm, *.tlb, *.tms, *.udf, *.url, *.vb, *.vbe, *.vbs, *.vbscript, *.vdo, *.wcm, *.widget, *.workflow, *.wpk, *.ws, *.wsc, *.wsf, *.wsh, *.xqt

If you are running a system on campus that connects with the SMTP gateway and need to receive a file of the above extension, please ask the sender to rename the file extension, compress the file, or send you a hyperlink to the file on the web.