Major League Baseball uses the initials "MLB" as their trademark. Can I register a trademark using MLBXX (Each "X" represents a different letter) if my service is also baseball related?
Like any good netizen, you have eliminated just enough information from your question to make it "safe" for publication on the Internet -- and not give away your idea. Yet, trademark questions of all kinds are judgment calls. When a trademark attorney answers a question, they think about the current situation and (probably unconsciously too) compare it to the prior cases that they have read about. I have read hundreds and hundreds (if not thousands and thousands) of cases. There is no rule at law that says if your trademark varies by 40% or more, it is not similar. The first goal of trademark law is to prevent consumers from becoming confused about who is providing them stuff. So, by trying to ask this question anonymously, you have neutered the facts so it would be impossible to even guess an outcome.
Your mark could be any of these: mlbXL, MLB09, mlBAG, MlBOY (lowercase L), MLBUT, MLBAT, MLBUZ. As you can see, each of these gives a different impression, or commercial impressions. So, some of these may be (confusingly) similar to MLB and some may not.
Likewise, simply stating "baseball related service" is not enough. Are you providing stats for MLB on your website? Selling instruction on playing baseball? Providing online retail for baseball equipment? Running an umpire's union? Each of these service runs in different channels of trade that could effect the analysis. Plus, there are several more factors (known as the "Du Pont" factors) that could be considered in a likelihood of confusion analysis. Then, there is the obvious problem that the MLB mark is famous, and, is probably accorded more protection.
Why would you want to incorporate someone else's mark? Major League sports of all kinds really value their trademarks. They spend a lot of time and money enforcing their marks. If you need to even "use" the MLB mark to refer to the MLB in a commercial context (and not even as part of your trademark), I would recommend talking to a lawyer first. The lawyer can help you understand how to fairly use the MLB trademark. Confusion would not be the only issue with using the MLB mark, dilution (because of the fame of the MLB trademark) could also be an issue.
So, if you want to avoid receiving a cease and desist letter from Major League Baseball, you should seek the advice of your trademark attorney. In some states (including Arizona, I'm not sure about Florida), the information that you provide to an attorney, even before you hire or pay them, may be protected by confidentiality. Having said this, do not provide confidential information before an attorney ask you for the information. Every attorney has a process for screening potential clients to avoid conflicts and protect existing clients. These "prospective client" laws seems unlikely to protect someone who just randomly sends confidential information to every attorney without invitation.