You speak of "off-shore" as though it represents some magically lawless region. As useful as that might be for these purposes, it doesn't exist. Your server will exist in some country. That country will have laws; those laws may or may not favor you. That country may or may not wish to refuse a US court order. Most countries are on fairly good terms with the US, and unlikely to say "no" to a request for evidence based on what looks like a standard legal investigation. Many of the countries not on good terms with the US rank pretty high on the list of places not well known for respecting rights, least of all privacy. And on top of all that, keeping a server outside the US does not render you personally immune to a US warrant if you remain in the US.
(Most of the above applies for s/US/$country/g as well.)
You're likely better off hosting a mail server here in the US, not actually storing any mail on it, downloading the mail immediately to a local mail store on an encrypted disk, and using encrypted email to protect new mails. That still won't render you immune to prosecution, but you might consider the consequences of refusing to decrypt a server preferable to the consequences of revealing the contents of your email. Or not.
I'm not a lawyer but I believe it would be very difficult for an FBI prosecutor to raise a case to international status and get a foreign judge to issue a subpoena. Most countries are very protective of spying on their citizens or corporations.
I think the main danger of using a foreign host would be that the NSA has more spying leeway with international traffic so they probably will have already slurped and archived your mail off of the wire without needing to ask your mail host. SSL is supposed to help with that but I have my doubts that that stops the NSA if they want to target you.
Or do the above, and then delete the mail. Not with the intention of destroying evidence of course, but because your hard disk can only store 1024 megabytes and you want to save disk space.
(Most of the above applies for s/US/$country/g as well.)
You're likely better off hosting a mail server here in the US, not actually storing any mail on it, downloading the mail immediately to a local mail store on an encrypted disk, and using encrypted email to protect new mails. That still won't render you immune to prosecution, but you might consider the consequences of refusing to decrypt a server preferable to the consequences of revealing the contents of your email. Or not.