E-mail Attachment Types Blocked by Towson University Mail Servers
With the increase in harmful e-mail attachments and viruses, the University (along with most other institutions and Internet Service Providers) blocks certain types of e-mail attachments to prevent clients from receiving infected e-mail messages.
There are several layers at which e-mail attachments can be blocked:
- Email Security Appliances (mail traveling between non-University addresses and students/faculty/staff)
- Outlook and Outlook on the Web (Exchange Online environment for students/faculty/staff)
- Microsoft Defender (or other virus scanners)
Email Security Appliances
Messages passing through the Email Security Appliances may cause a bounceback (non-delivery report) to be sent to the sender (with error “The file attachment(s) you sent have file extensions that cannot be accepted by Towson University's mail servers (such as executable or compressed file types)” if the e-mail message contained an attachment with any of these extensions:
- an executable, batch, script or installation file:
.exe .msi .bat .scr .pif .com .cmd .mst .vbs .vb .wsh
- zip and other archive files:
.zip .rar
- code libraries, drivers, etc.:
.dll .lib .sys .ocx .inf .vxd .reg
- web page or file links/bookmarks/shortcuts:
.url .lnk
- Files that are moved to Quarantine instead of blocked.
.docm
- The complete list of file extensions (which includes ones already mentioned above):
- ade adp app asd asf asx bas bat bat chm cmd cmd com com cpl dll exe fxp hlp hta hto inf inf ins isp jse* lib lib lnk lnk mde msi msi msp mst mst ocx ocx pcd pif pif prg rar reg reg scr scr sct sh shb shs sys sys url vb vb vbe vbs vbs vcs vxd vxd wmd wms wsc wsf wsh wsh zip zzz
Outlook and Outlook on the Web
Outlook and Outlook on the Web will also prevent the ability to open certain attachments. The symptom will usually be an alert at the top of the message window stating that a specific attachment cannot be opened.
The list of Microsoft 365 Outlook on the Web blocked extensions are listed at: https://support.microsoft.com/en-us/kb/2852113
The list of Outlook blocked extensions are listed at: https://support.office.com/en-us/article/Blocked-attachments-in-Outlook-434752e1-02d3-4e90-9124-8b81e49a8519
In general, these file types tend to be scripts and executable files.
It should be noted that these lists are subject to change. In addition, other e-mail clients may block different sets of attachments.
Bypassing Attachment Blocking
If it is necessary to send a file of any of these types:
- Make certain the file is safe to send. Many mail servers can still scan files and determine their true content, regardless of extension. Therefore, your file may still be blocked.
- Rename the file with another extension (e.g., .txt, .zip2), and inform the recipient that they must rename the file back to its original filename. This obfuscation will work for many file types, but usually not for executables.
- It's advisable to use only letters, numbers and underscores when choosing filenames. Special characters can sometimes be blocked by mail servers.
- If mail servers keep blocking the attachment, then it will be necessary to deliver it to the other party another way (e.g., host it on a website). Individuals may wish to use the File Delivery Service to send other blocked files or very large files (http://fds.towson.edu).