There is absolutely NO REQUIREMENT on these newer versions by KMime.

This is only done to keep in sync with the rest of KDE PIM.

--- kmime-19.04.0/CMakeLists.txt.old	2019-04-12 00:14:19.000000000 +0000
+++ kmime-19.04.0/CMakeLists.txt	2019-04-19 06:18:49.665558623 +0000
@@ -4,7 +4,7 @@
 project(KMime VERSION ${PIM_VERSION})
 
 # ECM setup
-set(KF5_MIN_VERSION "5.56.0")
+set(KF5_MIN_VERSION "5.54.0")
 
 find_package(ECM ${KF5_MIN_VERSION} CONFIG REQUIRED)
 set(CMAKE_MODULE_PATH ${KMime_SOURCE_DIR}/cmake ${ECM_MODULE_PATH})
@@ -22,7 +22,7 @@
 
 
 set(KMIME_LIB_VERSION ${PIM_VERSION})
-set(QT_REQUIRED_VERSION "5.10.0")
+set(QT_REQUIRED_VERSION "5.9.0")
 find_package(Qt5 ${QT_REQUIRED_VERSION} CONFIG REQUIRED Core)
 
 ecm_setup_version(PROJECT VARIABLE_PREFIX KMIME