After some testing, I can confirm that that long tag string you see in the sieve script points to the group. As long as you don't delete the group it will be a unique pointer to that group. For example,
the tag string remains unchanged under these conditions:
- You can add or remove members of the group.
- You can change the name of the group in the Settings>Contacts>Groups screen.
I can't find any security issues with these tag strings. For example:
- Each time you create a group with a particular name, the tag string is unique. If you delete that group and later create a new group with the same name it will have a completely different tag string.
- If you try using a tag string in sieve on a different Fastmail account, the tag string has no meaning in that different account (unless you have shared that group with other Fastmail accounts in a business).
Bill