Python imaplib fetch command: not working with 'RFC822' but works with 'RFC822.SIZE FLAGS BODY.PEEK[HEADER.FIELDS (From To Cc Bcc ...'

Question

This is a minimal example of fetching emails from IMAP server using Python imaplib package. (You can check a similar source code from the official documentation.)

import imaplib
import email

HOST="MY_AWOSOME_IMAP_SERVER"
USER="[email protected]"
PASS="awesome password"

with imaplib.IMAP4_SSL(host=HOST, port=993) as imap:
    # Log in
    print("Logging in...")
    resp_code, resp = imap.login(USER, PASS)

    # Fetch the last two emails
    
    resp_code, mail_ids = imap.search(None, "ALL")
    print(f"Response code: {resp_code}")
    for mail_id in mail_ids[0].decode().split()[-2:]:
        print(f"-> Mail {mail_id}")
        resp_code, mail_data = imap.fetch(mail_id, '(RFC822)')
        msg = email.message_from_bytes(mail_data[0][1])

        print(f"From   : {msg.get('From')}")
        print(f"To     : {msg.get('To')}")
        print(f"Date   : {msg.get('Date')}")
        print(f"Subject: {msg.get('Subject')}")
        print()

    imap.close()

• This code works well when I tested it for many email servers including gmail.com.
• But it doesn't work when I tested for my client's email server.

The engineer from my client's email provider suggests to change the second parameter in the fetch command.

resp_code, mail_data = imap.fetch(mail_id, 'UID RFC822.SIZE FLAGS BODY.PEEK[HEADER.FIELDS (From To Cc Bcc Subject Date Message-ID Priority X-Priority References Newsgroups In-Reply-To Content-Type Reply-To)]')

If I replace '(RFC822)' with a lengthy text 'UID RFC822.SIZE FLAGS BODY.PEEK[HEADER.FIELDS (From To Cc Bcc Subject Date Message-ID Priority X-Priority References Newsgroups In-Reply-To Content-Type Reply-To)]', then I have this strange behaviour.

• Now the code works for my client's email server.
• But it doesn't work for other email servers including gmail.com

What does this lengthy parameter 'UID RFC822.SIZE FLAGS BODY.PEEK[HEADER.FIELDS (From To Cc Bcc Subject Date Message-ID Priority X-Priority References Newsgroups In-Reply-To Content-Type Reply-To)]' do? A quick SO search shows one question containing this parameter, but I cannot understand why this parameter works for my client but fails for other emails servers.

Can anyone explain what this is about?