For non-commercial use, PGP is probably the way to go -
interoperable versions are available both inside and outside the US,
and scripts to integrate it with a number of popular SMTP mail
packages are available. For commercial use, Viacrypt PGP is available
in the US, but I'm not sure if there is a non-US version which can be
(legally) used commercially (this depends on the patent and licensing
status of the IDEA cipher, which is used in PGP for bulk encryption.)
While private communication is (for the moment) legal in the
US, other countries may have other ideas. In particular, I seem to
remember hearing of problems sending encrypted data across the borders
of France and Japan. You may wish to check on local laws.
The source code for RC4 (a commercial stream cipher) was
recently posted to the Internet. Since it was not patented, but kept
only as a trade secret, it *may* (note emphasis) be legal to use if
you have not signed an NDA with RSADSI.
Disclaimer: I am not speaking for my employer.