If the board has already contacted you that is is an issue, that is your answer, if you doubt it look it up in your state regulatory code.  I think its just hokey anyways.  

Yes, there are a lot of states that prohibit any variations of, abbreviations for, or creatively-spelled forms of "architect", "architecture", or "architectural", used by an unlicensed person in any context that might confuse the public into thinking that you're an architect.  Since you're using this "arch" prefix in the same places that you're calling yourself a designer you're conceivably creating that confusion. 

The board is the only and final authority in your state.  If they say it's a problem, stop using it or they may fine you.  If you want to argue about it then you should contact that board - but stop using that email in the meantime anyway.  Note that most boards do not actively search for these sorts of things, so most likely somebody reported this.  There are some people (even on this forum) who make this their mission in life.

The teeth, in this instance, is them flagging your account and causing problems with you actually becoming registered eventually.  I definitely agree, for software people (or any other profession for that matter), there's really nothing to stop them from using the term.

I also had an email from college with architecture in it, and stopped using it (without demand) when I realized (later in college) that it could be conceived as problematic.  I think my mom was telling friends that her son was an architect after my 2nd semester.  

Besides the potential to complicate or lengthen your eventual process of getting a license, some states will issue and chase down fairly substantial fines from unlicensed designers misrepresenting themselves as architects.  Amounts and enforcement vary widely by state.  For example California routinely fines unlicensed people $1000 for this sort of thing, though they usually give the courtesy of a warning first.  They enforce some cases to the point of intercepting tax refunds and garnishing wages to collect unpaid fines.  On the other hand some small states without dedicated architecture board admin staff or investigators only fine a handful of people per decade for anything at all, their fines top out at a few hundred dollars even for gross professional misconduct, and their most extreme effort at enforcement is one attempt by a county sheriff to deliver a demand letter. 

I guess if this is a thing you really want to be stubborn about, consider the likely magnitude of your state's response, wherever you are. 

Tell me the architecture board in question and I'll write a letter complaining to them myself ... and encourage other licensed architects from the forums to do so as well.. This is overreach and intimidation for no other purpose that to make the board feel like they have a purpose. Four letters in your email do not indicate to the public that you are an architect ... period, full stop. Unless you have been actively pursuing work based on the "credentials" expressed in your email address, you should have nothing to worry about. Unless you're misrepresenting their statement, they haven't even decided themselves whether this is an issue. You said they only alerted you that this may indicate to the public ... not that it does indicate to the public. Did they give you any direction to stop using that email, or to change it? Do they think people are looking for architects by searching email addresses with "arch" in them? Ridiculous!

That being said, I wouldn't fight it if I were you. You are at their mercy unless you want to hire a lawyer and fight it. They can easily make getting your license an issue if they felt so inclined, not only in this state, but in others because you likely need to disclose any issues with other state's boards when applying for licensure. I'm not sure what they expect you to do about it at this point, unless they've given you some sort of direction in the email they sent you. Maybe all that is needed is to register using another email for the time being until you get your initial license.

If you did feel like fighting it ... I'd recommend instead to change the state you will be getting your initial license in to avoid this board before they make a formal issue of it or take any formal action. Later, once you've received your license in the other state, get reciprocity in the original state and they can't complain that your email address is misleading at that point because you would be a licensed architect. 

I would like to provide more details for my situation.

 I used this email only for logging in my NCARB account and LinkedIn account. Also, connecting NCARB staff and the board staff, nothing else, literally speaking, the staff from these two organizations are the only people I have ever email to with this email address. 

Plus, the "myname" part of this arch.myname@gmail.com email account is not my official name, it is actually the name my friends usually called me, which has nothing related to my official name. And my profile name of this email is my official name. My point is, if someone really wants to imply the public that he/she is an architect, it would be much better to keep the names consistent.

I'm guessing that the board did a keyword search within their system, since in early January I received an email from the board with subject of "ARCH in the email", the content was my name and NCARB id, and few minutes later, they sent me another email said "recall the previous one".

I still think the whole thing is bureaucratic.

Strictly speaking, it probably is a violation in most states (assuming you're in  the US), but states will vary wildly in enforcement.

Here in California, the Board would send you a polite "please stop that" letter and then forget about you. They would not follow up to see if you'd complied unless/until they received another complaint, but would come down pretty hared on you if you hadn't.