Can I put star (★) in my email subject?
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We got a request from our 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). Good thing is they asked us first if this is possible and if there are any potential problems with this.   

So now, I tried to send a test mail, and the mail 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 `your mails 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 (keeping in mind the "customer is always right" policy - which means I have to be polite and persuasive).

**disclamer**: I know this is not the perfect programming question, but I really don't know where else to ask, so feel free to correct and point me in the right direction.

Thanks! 