Best way to guarantee unique mail_id across partitions when using mysql

[Dave Dodd via amavis-users]

What is the recommend way of ensuring mail_id is unique across partitions
when using MySQL ?

We are using a week number for the partition_tag field.

We are considering adding unique indicies on mail_id in cases.

Is there a better or more correct way ?

Can we just put one on the first table that amavisd writes to ?

Is the mail_id unique enough as it stands to do nothing about it ?

-- Dave