"Voting procedures must be readily understandable by citizens; ii. voters should always have the possibility of voting in a polling station. Other means of voting are acceptable under the following conditions: iii. postal voting should be allowed only where the postal service is safe and reliable; the right to vote using postal votes may be confined to people who are in hospital or imprisoned or to persons with reduced mobility or to electors residing abroad; fraud and intimidation must not be possible. "
CoE (Venice Commission): Code of Good Practice on Referendums, para. 3.2.i

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Code of Good Practice on Referendums