Can I put star (★) in my email subject?

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My question was:

I got a request from my client that they want to add stars (★) to their email subject (They send these mails through the application we made as a part of bigger CRM for them).

I tried to send a test mail, and the email title is displayed nicely in my Gmail account, and I must agree with my client that it is eye catching, but what came to my mind is that this may be a spam magnet, so I googled about it but I can't find the actual "don't do this".

Generaly, my oppinion would be not to use it, but now I have to explain to the client why. My best explanation whould be there is a probability your emails will be treated as spam but I don't have the background for this statement.

Do you have any suggestions about what should I do?

 The rather extensive answer, by Kibbee, was:

The only information I could find is on the SpamAssassin page of how to avoid false positives. The only relevant part I found was this part.

Do not use "cute" spellings, Don't S.P.A.C.E out your words, don't put str@nge |etters 0r characters into your emails.

SpamAssassin is a very widely used spam filtering tool. However, simply breaking one of the rules (strange characters) alone wouldn't get an email marked as spam. But combined with some other problems could lead to your email being considered spam. That being said, if your email is a completely legitimate business email, it's likely that few other rules are triggered, and using the special characters wouldn't create a huge problem. That being said, you should probably try out a couple test emails on SpamAssassin and a couple other spam filtering tools in order to come to a better conclusion on the emails you plan to send out.

Additionally, there was also a good answer from Nathan White:

Simply explain to your client as you have explained to SO: you stated that the star made it eye catching: this doesn't directly mean that it will be treated as spam, but you could explain how that concept COULD be considered spam.

If the star is part of their branding, however, this could be quite a nice way in which your client expresses themselves.

Spam emails are becoming more and more like what one would consider 'normal', so I think they have trial it internally, test the concept.

Talk it over with your client - there is going to be no basis in hard fact with things like this, purely social perception.

And also by lukeA:

More and more retailers are using unicode symbols in their subject lines since a few months. Of course it's in order to gain more attention in cluttered inboxes. Until now, there has been absolutely no evidence that such symbols increase the likelihood of failing spam filter tests. However, keep in mind that rare symbols might not render (correctly) across all mail user agents. Especially keep an eye on Android and Blackberry smartphones, but also on Outlook. In addition, due to a Hotmail bug symbols will render much bigger in subect lines and in the email body within the web front end. In fact, they are beeing replaced by images. All in all, the star shouldn't make any problems. At least, if it's encoded correctly in the subject line. So, go for it.

Written by Nikola Brežnjak