Thanks for your comments. In fact there are two functions one can use as archives: 'followed contacts' and 'followed Conversations' ; the defect is that original messages are not deleted from the accounts, which tend to become uselessly immense.
The problem of format (.mbs) can be totally ignored if one follows a different very clean procedure, which, furthermore, does not flood the files dedicated to operamail in windows:
prepare a folder somewhwere (eg. on desktop) with some sub-folders for different addresses, and for any subfolder at least two sub-sub-foders one for messages in input and one for messages in output. You can create a subfolder any time you save a message of someone else. Once this is done (once and for all), one can follow this procedure:
select the message to be archived
select 'file' than select 'save as'
save the message in one of the the (sub)folders created in phase 0, and that's all; now you can delete the original message.
The message is in format mbs, but you can ignore this fact. If you can read a message, select on operamail 'file', than select 'open', go to the interested folder, select 'all files' and 'open'. The message is immediately open in operamail.
a. the messages are opened without images, but 'right button' and 'reload image', and image is reloaded.
b. since the header is not saved, you should first save the attachements, if any. The function 'save' of the attachements solves the problem: you can save them in the same sub-folder of the message, and then you can read them freely. If you want to maintain the attachments safely bound to the message, you need only to create for the message a sub-sub-folder, and to save message and attachements in it.
I apologize for my poor english, but hope all this is clear enough.