A large proportion of e-mail is spam, much of it pornographic or otherwise offensive, and malware-bearing messages (viruses and worms). The Forest Service blocks as much of this as possible without interfering with legitimate messages, which of course isn't always possible. The kinds of messages that are blocked include ...
messages pretending to be from the Forest Service, or from certain popular ISPs, but that are not sent from an expected relay host
unreplyable messages which are almost always spam or malware
e-mails containing dangerous attachments
e-mail from host machines or domains that are known to be spammers, based on publically-accessible blacklists
The Forest Services locally maintains a set of filters that block senders and hosts based on user complaints and reports of spam. Filters are applied at several stages of the e-mail reception process. The following attachment types are blocked at the SMTP mail gateways ...
.bat; .cab; .chm; .cmd; .com; .dll; .eml; .exe; .js; .jse; .lnk; .msc; .mtx; .nws; .pif; .rar; .reg; .scr; .shs; .sys; .vbx for any x, including .vb.; .wsx for any x, including .ws; .zip
Additionally, external e-mail messages can only be 5 mb or less. Because of the store and forward nature of SMTP mail, large messages fill the queues and are prone to fail, requiring multiple transmission attempts. This interferes with the e-mail transmission and reception process for everyone, not just the sender or recipient of the large message. This size limit includes the entire e-mail, not just the attachments, which includes the body, the header, and signature of the message as well as the attachments. FTP is a better way to share large files because it moves the data directly without intermediate queuing.
For technical reasons, when a filter is applied or the size limitation exceeded, it is not always possible to reliably send a reject or nondelivery notice to the actual sender of the message. Because delivery to the actual sender can't be assured, which would occur with much of the malware-containing e-mail, the Forest Service declines to send any notice of non-delivery.
Therefore, please be aware that if your e-mail is blocked for any of the reasons outlined about, you WILL NOT receive notifcation that it has failed to reach your intended recipient. When sending attachments or large files and uncertain if your e-mail meets the accepted criterion, please contact the recipient to make sure it has been received.