If the mail, electronic or otherwise, is in transit there are
ways they can intercept it and read it, some legal some not. But once the email has reached you, to read it they have to get access to where you have it stored. If you're under investigation for something they may be able to get legal authority to access and read the mail. If the sender is under investigation, they may be able to get authority to access and read the mail.
Now if they're investigating the sender and get a warrant to access mail you have, such court orders usually limit the access to ONLY the mails exchanged with the person they're investigating. And sometimes they limit them even further to only those on the subject they're investigating the person about.
As a traveller within the USA it shouldn't be much of an issue one way or the other, unless you're a travelling bomb maker. As an international traveller, it's much the same, but the US already has existing laws allowing them to fully examine any electronic device crossing the international boundary and to check ALL the data on it. One reason why some companies now have a few VERY sanitised notebooks for use by staff travelling overseas, they have no data on them and what they need to take is on a DVD or external drive with only the data for the trip on it. This is to limit access to any other company data.
