Having problems with POP3 setup

[X Tec]

Hello.

After exploring all possibilities I could with IMAP, I felt like it was time to give POP3 a try.
So tried the following:
set pop_user="$user"; set pop_pass="$pass"; set pop_host="pop://$user@$url" set pop_delete=no; set pop_last=yes; set mbox_type=Maildir; set folder=~/mutt/mail; set spoolfile=+inbox; set record=+sent; set trash=+trash; mailboxes +inbox +sent +trash; set ssl_force_tls=yes

But it's not working:

---It creates $HOME/mail/, but just "$HOME/mail/inbox is not a mailbox". Ctrl+O to this mailbox gives the same, until hitting 'G' key to fetch email.

---Fetching email with 'G' key just ignores the "pop_last=yes" setting because it always downloads all email regardless of locally read or not, even though other clients such as Outlook or Thunderbird don't do this mistake. So I don't think server doesn't support the LAST command...

---When trying to open the other mailboxes I just get the "not a mailbox" message, either with Ctrl+O or 'G', even if I manually create the corresponding directories

---A more general thing: what are Ctrl+O ("open mailbox") and '$' actually for?
On IMAP Ctrl+O is the only one updating the mailboxes; '$' just says "mailbox is unchanged" even if there are being new messages at that instant (I have tested it), and only does stuff only if there are messages marked for deletion
On POP3 both Ctrl+O and '$' seem to do nothing at all, and only 'G' downloads messages

---Finally, on IMAP, different email providers seem to have totally different ways to specify their subfolders for the variables $sent, $drafts...: "INBOX.sent", "[Gmail]/Sent Mail", etc. Then how am I suppose to find out which syntax each email service uses?

Thanks beforehand for your attention.