fortimail - default action

1- What does it means the default action in the antivirus profile ? what if I select "none" ? "see screenshot"

The purpose of the default action is to remove the need for the admin to define the action for each profile feature.  You set --Default-- and make the changes only to the defined --Default--- Action profile.   You can then either specify to use the default or override with a specific action profile.

If you specify to use the --Default-- Action profile but then set the Default to None, then no action will be taken on detection (i.e. don't do this :) ) .

2- What is the recommended version and is there any path to upgrade from 5.3.3 to the recommended version ?

Recommended version is either 6.0.2 or 5.4.7 depending on your hardware version.  Note that 6.0.3 is due out in under 2 weeks if you want to wait for the latest PR of 6.0.

Upgrade path can be found from the release notes e.g. from the 6.0.2 release notes:

5.3.3 

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5.3.10 (Build 643)

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5.4.4 (Build 714) (Required for VMware install only)

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6.0.2 (Build 0108)