All is correct on my system also a yum clean is make.

But the dovecot = 2.3.5.1-1 is MISSING on the repositories !!

Sorry, this is wrong, it's there. Have a look at: http://repo.dovecot.org/ce-2.3-latest/centos/7/RPMS/x86_64/2.3.5.1-1_ce/

dovecot-2.3.5.1-1.x86_64.rpm 25-Mar-2019 09:29 4649368

If yum doesn't work for you, download the rpm and install it with yum locally.

Yum like to update to the new packages dovecot-lua and dovecot-imaptest but
can't found the latest passed dovecot Version 2.3.5.1-1

The packages dovecot-lua and dovecot-imaptes 2.3.5.1-1 are found but not the
dovecot version??

[root@localhost ~]# yum install dovecot
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
* base: mirror.fra10.de.leaseweb.net
* epel: ftp.nluug.nl
* extras: centos.mirror.iphh.net
* nux-dextop: mirror.li.nux.ro
* updates: mirror.checkdomain.de
Resolving Dependencies
--> Running transaction check
---> Package dovecot.x86_64 2:2.3.5.1-1 will be installed
--> Processing Dependency: libclucene-core.so.1()(64bit) for package: 2:dovecot-2.3.5.1-1.x86_64
--> Processing Dependency: libclucene-shared.so.1()(64bit) for package: 2:dovecot-2.3.5.1-1.x86_64
--> Running transaction check
---> Package clucene-core.x86_64 0:2.3.3.4-11.el7 will be installed
--> Finished Dependency Resolution

Dependencies Resolved

============================================================================================================================
Package     Arch   Version   Repository     Size
============================================================================================================================
Installing:
dovecot     x86_64   2:2.3.5.1-1   dovecot-2.3-latest   4.4 M
Installing for dependencies:
clucene-core   x86_64   2.3.3.4-11.el7     base   528 k

Transaction Summary
============================================================================================================================
Install  1 Package (+1 Dependent package)

Total download size: 4.9 M
Installed size: 16 M
Is this ok [y/d/N]: y
Downloading packages:
(1/2): clucene-core-2.3.3.4-11.el7.x86_64.rpm   | 528 kB  00:00:00
(2/2): dovecot-2.3.5.1-1.x86_64.rpm   | 4.4 MB  00:00:00
----------------------------------------------------------------------------------------------------------------------------
Total     5.0 MB/s | 4.9 MB  00:00:00
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
  Installing : clucene-core-2.3.3.4-11.el7.x86_64     1/2
  Installing : 2:dovecot-2.3.5.1-1.x86_64     2/2
  Verifying  : clucene-core-2.3.3.4-11.el7.x86_64     1/2
  Verifying  : 2:dovecot-2.3.5.1-1.x86_64     2/2

Installed:
  dovecot.x86_64 2:2.3.5.1-1

Dependency Installed:
  clucene-core.x86_64 0:2.3.3.4-11.el7

Complete!