What the Comune certifies
A Certificate of No Impediment states that nothing in the records bars you from marrying. In Italy the Comune issues it, and in Messina that means the civil status office at the town hall. It rests on your own register entry, so the clerk will look at your birth record and your marital status. That is the ground floor for anyone asking how to legalize a Certificate of No Impediment.
Before the town hall prints it
Bring photo ID and be ready to say where and whom you are marrying, because some town halls print the partner's details on the face of the sheet. If you were married before, the record of how that ended has to be on file in Messina already. A divorce or a death recorded abroad often has to be entered at the Comune before a clerk will issue anything.
An administrative record
This is a civil status paper, so the Prefettura in Messina legalises it. The Procura della Repubblica is a separate counter for deeds signed before a notary and for judgments. Even a foreign divorce decree sitting behind your file does not move the sheet to the court. Consular legalization of a Certificate of No Impediment in Italy therefore runs through the Prefettura.
The other country's mission
Then the file goes to the embassy or consulate in Italy of the country where the wedding will be held. That clerk reads the Italian mark and adds a consular one. The embassy attestation of a Certificate of No Impediment in Messina is what the registrar abroad will want to see. Missions differ on forms, on hours, and on whether they hand the file back or post it.
How long it stays good
These sheets go stale fast. Many registrars abroad will only take one issued within a few months, and a few are stricter still. The clock runs from the day the Comune prints it, not from the day you send it on. So order it once the wedding date is fixed and the rest of the file is ready. Ask the registrar for their own limit in writing.
What you will pay
A €16 tax stamp (marca da bollo) goes on the Italian side, and the consulate charges its own fee after that. Count 10–15 business days to legalize a Certificate of No Impediment in Messina, or 3–5 where a rush lane is sold. Weddings run to fixed dates, so build in slack for holidays at both ends of the chain.
Ask the registrar first
Whether the consular stamp is wanted at all depends on the country holding the wedding, so ask the registrar there before you spend. A number of them take a single-step certification and nothing further. If the chain is wanted, confirm the required language with the receiving office in writing, and ask when the rendering should be bound in. Then the embassy legalization of a Certificate of No Impediment in Messina can be planned once.