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at last I figured out what the problem was by comparing files and db tables of a fresh 2.4.5 installation and my upgraded 2.2.4 to 2.4.5 site
there seems to be a bug in the upgrade procedure, the filed in the configuration table which contains information about smtp in the updated 2.2.4 to 2.4.5 db was declared as a text field while in the fresh 2.4.5 db it is an array
by changing manually the field type to "array" in the upgraded db and modifying consequently it's content, the mailer started working