Taking a step back for just a moment, I can see an actual suspect (one in which they have gone and gotten a warrant on) having a mail account that has mailed xxx number of other emails - if they really think their warranted suspect is a terrorist threat then they will want to look into the email accounts/gchat logs of those he emailed.
I think the concept of needing to do this is ok in certain investigations but it needs needs more judicial oversight with checks and balances. Even if it's a judge saying "what investigation is this attached to?" and tying it to a subpoena'd suspect. I'd also like to make sure that this is only used for national security issue. I wouldn't want this flipping over into normal run of the mill criminal activities. I'd even go so far as to say "If evidence is found in this way it's off limits" as evidence so it doesn't have the temptation of being abused.
Things are not as simple as they seems at first. - Edward Thorp