As we all know, the developers of Debian take their time to include updates to many of the packages available in the repositories, and now it is the turn of Icedove, an application that seemed to have been forgotten.
Icedove is a fork of Thunderbird, and they have just entered the repositories of Debian Testing the version 10.0.2, being finally replaced the version 3.1.10. As we all know, Thunderbird is in the version 11.0.1 and it won't be long until the 12.0. I guess as usual the guys from Debian have not updated Icedove to version 11 for some security or instability reason.
We only have to update if we already have the application installed. However, if you want the latest version of Thunderbird o Firefox en Debian, I recomend you this article which shows us how to make them the default applications for Surfing the Internet or Reading Mail.