There are some universally acknowledged rules on email composition: never leave the subject line empty and, by and large, avoid beginning with “Dear Sir/Madam” as it is considered too formal these days. After this, we must rely on our own correspondence style, which can be too casual for some or overly wordy for others.

Crystal for Gmail is a Chrome plug-in that guides you through composing the perfect email based on the personality of the recipient. First, Crystal combs through publicly available text on the web associated with the person in question – this could be anything from a LinkedIn profile to a blog post they wrote. Then a "personality detection analysis" in carried out in order to match them to one of 64 possible personality types.

As you compose an email the “Crystal says” button will yield a pop-up full of suggestions, eg “When emailing Adrian use emotionally expressive language”, or “Avoid the use of emoticons and exclamation marks when addressing Jane”. It might be clever guess work but when I checked with some colleagues they admitted their Crystal profiles were fairly accurate.

https://www.crystalknows.com/