Design + Content = Success
Modern postal services (including the Radario platform) offer you all the necessary elements and templates for creating a beautifully designed letter. So let's talk about the content. The first thing you should pay attention to is the subject of the letter. We're bore to death of this expression, but still, first impressions are half the battle.
The head of the letter should be given maximum attention, because it determines whether a user will open your letter. Undoubtedly, if you send a message to your viewers who have already bought tickets for your events, you can be sure that they know about you, and, at least, your letter will not be a surprise to them. However, this does not negate the fact that you should clearly, briefly and most interestingly formulate your proposal in the subject line. It may be with a spectacular and, perhaps, provocative title. But you should not abuse your imagination, too much pretentiousness can only alienate your addressee from reading the letter.
As for content, you do not need to get carried away. Remember that the size of your message directly affects the conversion. So it must be clear and relevant, otherwise it highly probable that the addressee will not finish reading the letter to the end. But, again, do not forget that you are communicating with ordinary people, not with officials or robots. Although, of course, the questions of stylistics is up to you, as well the manner of communication with your audience.
Always remember that the main thing in your letter is a call to action. It must be noticeable and obvious. For example, you want to give viewers the opportunity to buy a discount ticket or offer to leave a review of the past event they visited. Accordingly, everything in your letter should work for the person to click the appropriate button, whether it is to go to the purchase or form to fill.