It's not quite clear to me: Can these rules be applied to already existing mails at creation time?
This is convenient in Gmail and when I last looked at fastmail in 2014 this wasn't possible.
This was the support answer:
"I am sorry, but our filters only work at email delivery time. You can't really use them on emails already delivered to your account.
However, to work on already delivered emails, I would suggest you use our 'Search' feature. You can search for emails matching specific criteria, and then select them and move them en masse to a different folder, or even delete them."
This works but makes migration/reorganizing a bit annoying
I enabled the new rules that support labels and opted into the beta. If you create a new rule that automatically applies a label, you can choose to apply it to "All matching conversations now" or "New matching messages on arrival."
> It's not quite clear to me: Can these rules be applied to already existing mails at creation time?
A subset of the new rules system can be applied to existing mails when the rule is created, at least when the rules are used with folders. I've not tried the new label system, but I think the folder/label processing comes after the rule matching so I'd expect it to be the same.
Once a rule is created, there does not appear to be any way to change whether it is in that subset or not.
Consider a rule that filters on who the mail is from. On the old rule creation screen, you could select whether to use the from email address or the from name in your condition. For email address you could check for containing, beginning with, or ending with a string, matching or not matching a glob pattern, or matching or not matching a regular expression. For a from name, you could check for exact match, contains, or glob pattern.
On the new rule creation screen, there are no longer separate from email address and from name options, and there is no choice in how it matches. You just enter a string--I'm not sure how exactly it uses it.
If you want the older, more flexible conditions you can get them by clicking "switch to no-preview rules". That gives, as far as I've seen, the same condition options that the old rules did. The cost, as the name "no-preview rules" suggests, is that you do not get a preview of what messages the rule matches.
When you hit "continue" on the rule creation screen for a "standard rule" (that's their name for non-no-preview rules), it brings you to a screen that shows all your existing mail that matches the rule, with a label showing what folder it is currently in. From there you can ask to further edit the selection criteria. Once you are happy with the messages it is selection, you can tell it to go ahead and create the rule.
That brings up the screen to specify the rule actions. On that screen there are check boxes to apply it to existing messages (the ones you saw in the preview) and to apply it to new messages.
When you create a no-preview rule, there is as you would expect no preview. There are also no check boxes for existing/incoming. It applie to incoming, and only incoming.
If you create a no-preview rule, and later edit that rule, there is no way that I've found to change it to a standard rule. If you create a standard rule, and later edit that rule, there is no way that I've found to change it to a no-preview rule.
Fastmail also lets you create rules by editing the sieve code text. The sieve code contains various rules generated by Fastmail internally and all the rules created from the rules you make in the Fastmail GUI. Between each section of such rules they have a text area where you can enter your own rules.
I don't know if there is any way to get it to do the preview and apply to existing thing with rules entered via those text areas. If there were I'd be tempted to stop using the rule making GUI and just do all my rules myself in sieve.
This is convenient in Gmail and when I last looked at fastmail in 2014 this wasn't possible.
This was the support answer: "I am sorry, but our filters only work at email delivery time. You can't really use them on emails already delivered to your account.
However, to work on already delivered emails, I would suggest you use our 'Search' feature. You can search for emails matching specific criteria, and then select them and move them en masse to a different folder, or even delete them."
This works but makes migration/reorganizing a bit annoying