Blocked and Potentially Dangerous Attachment Types
To increase security and protect users from malware threats (such as viruses, spyware, ransomware, etc.), some types of attachments are automatically blocked in Microsoft 365 mail at MU.
Executable files that may contain malicious code are primarily blocked in this way.
List of blocked attachment types
ace, ade, adp, ani, apk, app, application, appref-ms, appx, arj, asp, aspx, bas, bat, bgi, cab, chm, cmd, cnt, com, cpl, csh, deb, dex, diagcab, dll, elf, exe, fxp, gadget, grp, hlp, hjp, ht, hta, htc, img, inf, ins, iso, isp, its, jar, jnlp, job, js, jse, kext, ksh, lha, lib, library, lnk, lzh, macho, mad, maf, mag, mar, mas, mat, mau, mav, maw, mcf, mda, mdb, mde, mdt, mdw, mdz, msc, msh, msh1, msh2, mshxml, msh1xml, msh2xml, msi, msix, msp, mst, msu, ops, osd, pcd, pif, pl, plg, ppa, ppam, prf, prg, printerexport, ps1, ps1xml, ps2, ps2xml, psc1, psc2, psd1, psdm1, pst, py, pyc, pyo, pyw, pyz, pyzw, reg, rev, scf, scr, sct, shb, shs, sys, theme, tmp, uif, url, vb, vbe, vbp, vbs, vhd, vhdx, vsmacros, vsw, vxd, webpnp, website, ws, wsc, wsf, wsh, xbap, xll, xnk, xz, z
The recipient does not receive the e-mail, and the sender is informed of its undeliverability. When sending a message with a blocked attachment from the Outlook desktop version, the following warning appears:
First part of the undeliverability report:
The error code and the reason why the message was not delivered are also included in the email (section "Error details"):
550 5.0.350 One or more of the attachments in your email is of a file type that is NOT allowed by the recipient's organization.
To share such files, you can use other services available to MU users:
- OneDrive – How to upload, share and comment on files in OneDrive
- SharePoint – How to share files in SharePoint
- IS File Depository v IS MU
- Filesender Cesnet (suitable for sharing large volumes of data - up to 1.9 TB)
Emails containing certain types of risky attachments (e.g. MS Office files with macros) will be delivered to the recipient's mailbox, but will also be marked with a warning about the potential risk.
List of potentially dangerous attachment types
dotm, docm, xlsm, sltm, xla, xlam, xll, pptm, potm, ppam, ppsm, sldm, html, htm
Attachments Containing Compressed Files (zip, 7z, rar, …)
For attachments containing compressed files, a warning about potentially risky content is inserted automatically, regardless of the archive contents.
List of compressed file formats
gzip, zi, zipx, tgz, tarz, bzip2, bz2, bz, 7zip, 7z, tar, rar, zip, gz