Is it legal to send emails having hyperlinks to other people's content?

Is there a copyright infringment problem if you send an email with links pointing to other people's content.

If you are talking about linking, and not about use of content in your email, then you are probably talking about deep linking and the legal precedents documented on Wikipedia would be of interest:

Ticketmaster later filed a similar case against Tickets.com, and the judge in this case ruled that such linking was legal as long as it was clear to whom the linked pages belonged. The court also concluded that URLs themselves were not copyrightable, writing: "A URL is simply an address, open to the public, like the street address of a building, which, if known, can enable the user to reach the building. There is nothing sufficiently original to make the URL a copyrightable item, especially the way it is used. There appear to be no cases holding the URLs to be subject to copyright. On principle, they should not be."

IANAL, but my guideline is always this: would I be pissed if someone else did this with my content?

If you give good attribution / credit, like you would want someone else to do when they circulate your content, then you'll probably be fine. Ask the above question honestly, and treat others the way you want to be treated.

I always use verbiage like, "recommended reading", or "you'll really like this post by _. Be sure to subscribe to their blog". It's unclear from your question the context in which you are linking to others' content. Just be sure to give proper credit. The internet is all about sharing quality information. Better yet, write a blog posts with the articles you're recommending, and link the recipient to that instead.

In matters of law, the answer often lies elsewhere...

In your case, what really matters is: would the site you are linking to enjoy suing you? If the answer is yes, they can make your life miserable, even though you'd prevail in court eventually, if you could afford it.

Bottom line: play nice, be careful and it will limit your legal exposure. But you can't rule out 100% the risk of a [frivolous] lawsuit. So you need to understand the dynamics of your space.

If your newsletter is essentially copying and destroying someone else's business, you are likely to get in trouble. It's that simple.