Several of our customers have reported that if they used DOWNLOAD_URL placeholder in the content of the client email, links were visible only as plain text and not clickable. This makes it difficult for the customers as they are used to clicking on links but in this case they would need to copy the link and then paste it to their browser to access the Order Details page so they can download their tickets. This issue mostly happened with customers receiving their emails with Yahoo Mail which, reportedly, has an issue with "problematic" URLs in the content of an email and unfortunately, the definition of "problematic" is known only to them. However, we cannot exclude the possibility the this might be happening with some other email providers as well.
Now, there are two ways to tackle this; first is not to use DOWNLOAD_URL placeholder at all and use ORDER_DETAILS placeholder instead. This placeholder will display all the details about the order including a table with purchased tickets and direct download links for each of the tickets. We've tested and can confirm that links in that table are completely functional with Yahoo Mail.
Another way to resolve this problem is to use DOWNLOAD_URL placeholder as a href attribute (this might sound complicated but it really isn't). So, for example, head to Tickera Settings -> Email tab and within the content of Client Order Completed switch the view to Text (not Visual) and enter something like this:
You can download your tickets <a href="DOWNLOAD_URL">here</a>
This will result in having a sentence You can download your tickets here in the content of an email where the word here will be a hyperlink to the Order Details page. However, if you are utilizing this solution, once you have entered this to the content of email and saved changes, please check if WordPress automatically added "http://" as prefix to the placeholder. If that happened, URL won't work properly so you will have to manually delete "http://" and save changes again. Again, please do this within Text view rather than Visual view as it is impossible to save changes to the content which has a href attribute without WordPress adding "http://" prefix automatically.