Just be aware that most (all?) Mac mail clients insist on downloading ALL of your emails. I would like to keep Outlook installed on my Mac for the odd occasion I need access to work emails, but it insists on downloading and keeping an entire year of email for those odd occasions. Why Macs don’t have the feature that most (all?) mobile clients have is beyond me.
Old school expectations, I guess. Mobile means “limited”, even though you can get 256 GB Macs and 1 TB phones. 
As a Windows user as well, this annoys me as well, especially as Outlook for Windows has this feature.
I’ve started using the online web app for Outlook more and it’s not so bad.
Note that Thunderbird can limit the number of messages per folder downloaded, which is a similar feature. However, it’s not enabled by default.
You can change that in Accounts - Server Settings: When I delete a message.
You shouldn’t have to do that yourself though. Mozilla Thunderbird already seems to handle Gmail, Outlook, and Yahoo! Mail correctly, so why not iCloud Mail too?
I’ve found it doesn’t handle them all 100% correctly out the box. For Outlook, it tries to send the archived email to Archives, not Archive.
Believe it or not, but this might be on Apple’s side instead of Thunderbird. Depending on the mailserver, there are settings how to treat certain ‘system’ folders. I know this exists for Dovecot.
So if Apple doesn’t want to support this, you should change it yourself in your mail client.
Keep in mind that Apple isn’t very well known for supporting open standards. They mainly focus on their own ecosystem. So if iCloud mail is working with Apple Mail, all is fine for them.
Something must’ve changed then… 
I’ll try contacting their support team.
I hope that Apple will eventually do something about it!
I’m a current Mac Postbox user with an extensive set of rules for incoming mail. Is there a way to move from Postbox to Thunderbird without having to recreate them? Thanks!
If there is I did not discover it when recently moving to TB 115. I, too, had a rather extensive list of mail filters but I merely recreated them in TB.
That was worthwhile because for me TB 115 has some substantial advantages over Postbox. In particular:
I appreciate those points may not be sufficiently significant for you to outweigh the move, of course.
Stephen
BEWARE: This has not been tested!
You could try to transfer your rules by coping the Postbox rules from Library > Application Support > PostboxApp > Profiles > [your profile folder] > ImapMailFilters > [account folder] > msgFilterRules.dat
to
Library > Thunderbird > Profiles > [your profile folder] > ImapMail > Profiles > [your profile folder]
However Postbox does offer options in the Message Filters that are not available in Thunderbird. Like Play Sound and Run AppleScript. So I don’t know how compatible the msgFilterRules,dat files are between PB and TB.
I’m trying to import my Postbox local folders into Thunderbird but with no success. Can anyone please point me in the right direction? Thanks in advance!
So far as I recall I simply copied the relevant files from the Mail → Local Folders folder in Postbox into the same location in my Thunderbird profile then restarted Thunderbird and everything was there. The files you need are those like "Sent”, "Archives”, "Drafts”, etc.—i.e., without any file extension.
Stephen
Please correct me if I’m wrong, but it looks like Thunderbird won’t work well with Microsoft365/Exhange accounts. Is that correct?
You’ll need to install this extension in order for Mozilla Thunderbird to support Microsoft Exchange.
I do think that people over Postbox are now at work forking the Thunderbird code base for their own product for version 8 of their client.
I have too a lifetime Postbox license but keep donating each year money to the Thunderbird and Firefox folks to support the development.
Thunderbird is with me since the death of Eudora and my switch to GNU/Linux (first) and Mac OS X (Tiger and up-to-now). I am storing ~24 years of emails in it•
That’s interesting: thanks for that titbit!
Stephen
Ok I took the bait and installed Thunderbird. 
My thoughts are that this seems to be another app where email goes to die. I found no way of sharing email to other apps like Omnifocus, Fantastical, or Devonthink. No 1 click create PDF option is also a deal killer for me. Did I miss some config menu or is this accurate?
Full disclosure: I am a happy Airmail user.
Last time I tried copy and paste were working in TB. 
I think this was an assumption. Not a fact.
I tried the new Thunderbird for five months, and I mostly liked it, but the search capability was disappointing. I often find myself wanting to search the body of messages–like looking for a dollar amount of an online order because I want to add some details about what I bought to my accounting system–and I found Thunderbird to be surprisingly poor at this. I tried all the search options, and I repaired folders and rebuilt the index more times than I can count, but nothing worked. I could still find the information I needed by searching more generally (e.g., the vendor’s name) and reviewing individual messages, but I decided to reevaluate.
For what it’s worth, I moved back to Apple Mail for all my accounts, and it’s been great. My needs are admittedly modest; the volume I receive and send is pretty low, as is the number of messages I save. (I recently deleted ~10k messages from Gmail, leaving me with about 30 messages I wanted to save, and I did a similar pruning on my other accounts.) My use of folders is often temporary, like corralling all messages for planning a trip and deleting them once the trip is completed. My email life is simpler now, and I’m not missing the keyboard shortcuts to move messages to folders that I used to love in Postbox, or the fine-grained control afforded by MailMate. So for now, I’m back to basics, but it was nice to catch up with Thunderbird for a bit.