As far as I’m concerned, single opt-in isn’t even an opt-in at all. Its like RAID0 which stands for Redundant Array of Independent Disks, but there is no redundancy.
Single opt-in means that someone submits an email to be included in a mailing list, as opposed to someone gathering emails to put together a mailing list. The problem with the single opt-in is that the person submitting the email isn’t necessarily the owner of the email!
Nowadays most network managers require a double opt-in process, whereby someone submits an email to a mailing list, and then the mailing list sends out a confirmation email to the owner, asking them to verify that they were the one who requested inclusion to the list.
Seems obvious to me that every mailing list should implement double opt-in procedures.