There are occasionally situations where the sender and receiver are using different implementations of the same method which are incompatible. In these cases, the content is usually salvageable though file information may be lost. There also is an emerging base for Microsoft Exchange. We should probably worry more about the emerging systems than the legacy systems. Pine and ELM both have MIME support available in recent versions.
Most packages are pretty good at sending simple text messages between LANs and packages.
There has been some talk of an 8Bit based SMTP, but that would require significant upgrades to the network infrastructure to be successful. X400 is an alternate mail protocol that has robust attachment handling, but other issues including arcane addressing have limited it's use in the United States. The main reason is legacy investments. Several years ago e-mail was being implemented predominantly in LAN environments. It usually does but not always.