Yes ReetP, first of all i decided to have a backup using dd command. Is it quite worth or another backup option that would be better support for sme server restoration in case of failure?
Yup. Use the SME server backup (and read the wiki carefully on use).
Disconnect the server from the network so it stops collecting any mail etc, run backup from the console, and then 'restore from backup' when you reinstall.
Should work.
dd will copy all your existing partitions etc. with no raid partitions, so dd restore will put you back where you started, which is not what you wanted.
PLEASE go and have a good read about all this and educate yourself about how these things work.