professionalism - How do I tell my boss to not use Comic Sans as ...

Because they're my superior, I feel awkward bringing it up, but I think it's something that needs to be brought up. How do I word it to my boss so they are not offended but also change their typeface to something more professional?
The top answer to What is wrong with Comic Sans? on the graphic design site is written with remarkable restraint. Unlike most things written on Comic Sans it is pretty non-offensive, while still getting the point across
To answer the question title specifically, You don't. You can try all of the suggested answers below to convince him to change his ways on his own, but in a corporate setting, you cannot force your boss to do anything he doesn't want to - and if he wants to use Comic Sans and will not be convinced to change his font type, you should not risk losing your own job over it.
Why do you think that 1) Everyone who gets the email sees what font it was composed in, rather than the font they prefer to read? and 2) Of those who do see the email in its original font, what percentage do you think actually cares? Oh, and #3: Why on earth do people use anything but ASCII text in email, anyway?
I started to laugh, but then remembered the time when a student said she couldn't take my business class seriously because I used Comic Sans for a font. (For the record, it was Apple's Chalkboard, but still...)
Just as an aside, it's interesting how country cultures can differ. In a country like the Netherlands it would be a total non issue to casually mention something like this to your boss and it will be seen as a sign that you actually care. A reaction like "you should feel awkward bringing it up" is nearly impossible to imagine over there.
I read an interesting article the other day, which said that Comic Sans has a mixed reputation in the business. I wonder what our client thinks about it, perhaps they don't care, but just thought I should let you know.
This gets the message across, doesn't sound condescending, ensures that your manager doesn't lose face, and also saves you from the awkwardness of "recommending" a font to your superior, because you are putting the decision in his hands. If your manager is a reasonable person, it is hard to imagine him taking offence to this, especially since you offered to first understand his reason rather than just pass judgement. :)
In addition, it also lets you save face if the manager has some genuine reason. For example, if the font was the default on his computer and he hasn't figured out how to change it, or the client "likes" that font!
PS: As David K pointed out in the comment, you might want to keep such an article handy. Make sure that it is from a reputed source, not a random blog, and don't send it to your manager unless he insists.
Consider sending just one email though. One danger of splitting the message is that he can't use the information in the second message to influence his answer to the first message. You first ask him to confess he adores the font, and then tell him the entire world thinks otherwise. If you first tell him about the reputation, only then does he have a real chance to save face and, instead of admitting he loves the font, being aloof and blaming it on his secretary :-)
@JoeStrazzere: I don't disagree, but it is a pity these non-confrontational openings are sometimes necessary for bringing up such a trivial matter. If it were me using the font with no idea of its reputation, I'd prefer a direct "Stop using Comic Sans, it's unprofessional," after which we both move on to other things.
@MarcksThomas I would prefer direct communication too, but I found out over time that the threshold for what offends them is very low for a lot of people. Over time, I have gained increased mastery over this practice of sugarcoating the most mundane things, although it makes me cringe each time I do it. :-)
There is a website Comic Sans Criminal, which allows you to sent your boss an anonymous e-mail informing about the inappropriateness of this font. If you do choose an appropriate timing and are not known as a typography expert at work, it should not be relatable to you.
Of course, this at most works for this particular problem and if your boss chooses to switch to Papyrus, you need a different solution. Also, you are probably burning the bridge to bring up this topic in a non-anonymous way, should this approach not work.
Note that I have not tested this service yet and thus do not know whether the mail contains IPs or other identifiable information. Neither do I know what they do with those e-mail adresses. Use at your own risk and better test it with a mail to yourself first.
Finally note that sharing your boss’s email may be or may be considered a breach of rules.
I am not so sure it is a good idea to talk to your boss anonymously. This approach might get around this problem, but what about the next time he needs to have an awkward conversation with his boss? Awkward conversations are unavoidable in the workplace, you will run into them sooner or later, especially as you climb up the ladder. It is important to learn how to deal with them directly, than from behind the anonymity of internet.
Is there a reason to avoid directly asking the question? If you have regular one-on-one's with your boss, you could ask him about this during one of them (if you aren't having them start!). He may simply be unaware of how a typeface can affect the perception of the message.
I see no need for anonymous communication. This is an opportunity to show you are looking out for the company image and want to help.
You could start with something like:
Can I ask you a question about the font you use in email messages? I think you're using Comic-sans and in my experience it can carry a negative connotation, especially in a business setting. This is an area I'm interested in and I'd be happy to help select a strong font to carry our business message to our customers.
You could also come prepared with the website mentioned @Wrzlprmft's answer. And if you do go the route of wanting to suggest alternatives that are not "Times Roman", come prepared with those too. Bring solutions to the table!
Really the best way to approach your superior about something awkward is either make it a meeting or take them out for lunch. Believe it or not, bosses are people too and they do make mistakes as well. Ask him if you can treat him to a lunch, which will not only help your personal reputation but put him in a setting where he doesn't have his guard up, like most people do at work. If you want to have a meeting, send him an email to schedule something and say you wanted to talk about a few general things that are of concern to you.
The Conversation
There's nothing better than being blunt. Truly, there are two ways to go about telling him: professionally or comically. Going the professional route would mean explaining to him all the details of why you're concerned and really showing him that you care about this. The comedic route would result in a small joke that he may or may not brush off by the end of the work day. Either way, do whatever you feel more comfortable with. It's perfectly ok to express any concerns you have in the workplace.
What Not To Do
Definitely do not tell him that the font is unprofessional. He will get upset if you insult him. Rather, say that you think that this is petty, but a better font might make the emails look cleaner. Compliment him on his writing or such before you bring this up.
Never start a conversation by saying that what you're about to say is useless or stupid. It seems to be a habit of people to degrade their topic of discussion before saying it so that they don't seem ridiculous. Make sure it is known that you really care about this topic.
I would tend to avoid some of the the possibly over-cautious approach of these answers. Of course, subtlety and wit is very important in a corporate environment, and naturally your job is something you want to be very careful with - but brevity is the soul of wit and he/she will appreciate your honesty.
Think of it this way, he's making a mistake - and he's unaware of it. One can either tell him, risking animosity over negative feedback, or leave him to keep making that mistake and make a fool of himself. He's (hopefully) not a tactless moron and he'll appreciate your courage to step in and try and help him out.
It's very important that you exercise compassionate speech and comradery in a situation like this, adhering strongly to the principle of "constructive criticism". So long as you can stay away from patronizing him or treating him with disrespect he'll probably be very grateful that you decided to inform him.
If it were me I'd find a time where we were both together under immediate pressure, and say something like
Hey, I'd been meaning to talk to you actually.
I'm a little concerned that a lot of people don't see the font you use in your emails as being appropriate for corporate communication - there's a general consensus that it's a kind of silly font. While I don't think any of us are in any way offended, this idea is pretty widespread and I'm just concious that you mightn't know of this. If you'd like I'd be interested in helping pick a font to better represent our business ...
It might seem childish or blunt to some, but in my own experiences I've found that simple honesty rarely goes amiss - speak your mind in a respectful manner at an appropriate time and there's very little to go wrong :) Good luck!
In the end it will come down to who the boss is, who the other party is, and their relationship. I don't deny that there are people out there who would take that badly - but if the boss in question is a stable, grounded, man or woman, then honesty is always as sensible path.