Thank you. I did it again and again but there is no doodlebomb to send after creating a new letter and installing it via HBL.
I think that is because of the doodlebomb that I got sent. Normally, there is a date under its icon. In my case, it is written "…20255". If that is the date of my doodlebomb, I won't be able to create a newer letter than this (unless we want to wait 18238 years
) because the doodlebomb that I got sent is behind that new letter after restarting the system / trying to install doodlebomb in HBL.