If someone is trying to send you an e-mail and they are receiving a bounce back message stating that their e-mail was rejected due to containing attachments with disallowed file extensions, this means that they've tried to send you an e-mail that contains an attachment with a file extension that is considered to be dangerous or executable. They are commonly used to distribute malware. Some examples include:
.eml
.exe
.com
.msi
.vbs
.pif
.reg
.vbs
.vbe
Instead of sending such files by e-mail, please have the sender provide you with a link to download the file from a secure location (such as a cloud sharing account, for example).
For .eml attachments:
Some older or misconfigured mail apps may attempt to send forwarded e-mails as a .eml file attachment. For e-mail forwarding, please ask the sender to forward the e-mail inline, rather than as an attachment (which creates a .eml file and will be rejected). The process to do this is entirely dependent upon the app they are using, and isn't something we can assist with. Most apps default to inline forwarding, but some older or misconfigured ones may try to forward as an .eml attachment. That will need to be addressed by the sender.
