The current gnu-javamail seems to be a CVS snapshot from 2004. A 1.0 release is currently available.
Created attachment 67983 [details] patch against gnu-javamail-20040331.ebuild A preliminary attempt at an ebuild. One significant thing is that it requires =gnu-classpath-inetlib-1.0, where there is currently a 1.0 and 1.1 release, but it isn't currently slotted.
Fixed. Thanks for reporting.