Is it hearsay?
Hearsay is: (1) a statement; (2) made outside of court; (3) offered as evidence; (4) that the out-of-court statement is true. But statements made by the adverse party in the legal proceeding are not hearsay. See Federal Rules of Evidence, Rule 801.*
In this case, we have a double-hearsay problem, because you have the lawyer making a statement, and then someone forwarding it to you, essentially making a statment that the lawyer made a statement. So presented as evidence, you'd be saying, "X said that Y made statement Z."
So to see if that's hearsay, you run the hearsay analysis on both statements, asking the court to accept both that the forwarding party sent the forward, and that the lawyer said the things in the original e-mail.**
Statement 1: The forward is an out-of-court statement that the lawyer said the things in the forwarded message, and you're trying to prove that the lawyer said the things in the forwarded message, so that probably puts you in the hearsay category. The question becomes who forwarded it to you. If the organization forwarded you the e-mail, then it's not hearsay.*** If it's from the lawyer's brother, then it's probably hearsay.
Statement 2: The original e-mail is an out-of-court statement that the lawyer holds a certain opinion. If you offer it as evidence, the hearsay determination turns first on what you're trying to prove. If you're trying to prove that the lawyer held the opinion that he laid out in the letter, then we have to ask whether the lawyer made the statement on behalf of an adverse party in the litigation. If the answer to either of those is "no," then it's probably not hearsay.
On the first question, for instance, if you're trying to prove something other than what opinion the lawyer had, it wouldn't be hearsay. Imagine the following e-mail:
You've asked for an assessment of The Organization's liability under the Widget Control Act in light of our bad fourth-quarter financial reports. The Act imposes a $100-a-day penalty on organizations that fail to include suitable safety devices on their widgets. The law went into effect on August 20, and we still don't have safety devices, so we could currently be held liable for $400, with that amount increasing every day. But the Statewide Widget Control Authority isn't up and running yet, and our widgets aren't likely to injure anyone, so you may want to take that into consideration when deciding how aggressively we want to pursue compliance. Similar circumstances allowed us to slow-walk our implementation on previous compliance programs.
If you wanted to prove that the lawyer believed the company was liable for $400, that would not usually work. But if you wanted to prove something else -- that the company knew about the Widget Control Act, that the company knew it didn't have safety devices on its widgets, that the company knew it was required to install them, that it had a profit motive for noncompliance, that it has a pattern of engaging in this behavior -- you could do that without having a hearsay problem.
On the second question, it matters whether the lawyer is speaking as an employee or attorney for the organization. If so, the connection is close enough that you'd be able to offer it without it being hearsay. But if he just sent that e-mail to a friend who forwarded it to you, then the connection is severed, and it may be excluded as hearsay.
Is it admissible?
Determining whether a statement is hearsay is only a first step. Being hearsay is not the same as being inadmissible, and not being hearsay is not the same as being admissible.
Even if the statement is hearsay, there's a long list of exceptions to the rule against hearsay. If a hearsay statement falls into one of them -- excited utterance, dying declaration, then-existing mental state, etc. -- then it may be admitted just the same. See Rule 803 and Rule 804 for a full list.
And if the statement is not hearsay, it could still be excluded for a long list of reasons. Based on what you've provided, I can imagine valid objections for a variety of reasons, including:
The evidence is protected by attorney-client privilege;
The evidence is protected work product;
The evidence is not relevant;
The evidence is substantially more prejuicial than probative;
The attorney is not qualified to offer an expert opinion;
The evidence has not been sufficiently authenticated;
*This analysis applies the Federal Rules of Evidence. Most state courts use basically the same rules.
**It may be that there were other people forwarding the e-mail between X and Y, so you'd have to run the same analysis for all of them.
***I'm assuming that you're speaking of a scenario in which the organization is an opposing party in litigation.