Here's a little IMAP-client program in Go.

• It is based on the Go "imap" library that implements the IMAP client protocol
• It is a more complete implementation that is capable to sync your online and off-line mails repeatedly without re-downloading the old messages, or to sync two mail folders from two different cloud mail accounts into a single mail box file.
• Just FTR, to send each message from within the mail box file as separated email to another (cloud) mail account, check out formail from the procmail package. I.e., you don't need to pay $$$ to sync messages between your different cloud mail accounts.

$ cloudmail 
Usage: cloudmail [global options] <verb> [verb options]

Global options:
        -v, --verbose Be verbose
        -h, --help    Show this help

Sub-commands (Verbs):
    list      List mailboxes
    fetch     Download mailbox
    sync      Sync cloud mail folder to existing mailbox file

Verbs:
    fetch:
          -f, --folder  Mail folder to fetch (*)
          -t, --trackid Track message Id
              --wy      Within years, only to fetch mails within this number of years
              --wm      Within months, ditto for months
              --wd      Within days, ditto for days
    list:
    sync:
          -f, --folder  Mail folder to fetch (*)
          -t, --trackid Track message Id
              --wy      Within years, only to fetch mails within this number of years
              --wm      Within months, ditto for months
              --wd      Within days, ditto for days

To list all Gmail labels:

cloudmail list

To download all messages for one label:

cloudmail fetch -f my-label

To download all while tracking each downloaded messages:

cloudmail fetch -f my-label -t

And to restrict downloading to messages within recent two months:

cloudmail fetch -f my-label -t --wm 2

Just for verification, the next example will redo the messages downloading, but this time will get recent six-month messages instead, to show that repeated downloading will not cause duplicated messages:

cloudmail sync -f my-label -t --wm 6

To configure the program, create a file called auth that contains IMAP address, then the username and password (on one line each). Example setup has been provided in the auth.example file:

imap.gmail.com:993
userid
passwd

• The downloaded mail box file will be named using the provided Gmail label (e.g., my-label) with the .mbox extension (e.g., my-label.mbox). To view it, use mailx -f my-label.mbox.
• If message-tracking is enabled, a message-tracking file, named using the provided Gmail label (e.g., my-label) with a .yaml extension (e.g., my-label.yaml), will be created/updated as well. It contains all the Message Ids from the downloaded messages. One Id for each message in sorted order. The Message Id is part of the RFC822 standard from mail header that uniquely identify an email message. They make re-downloading or even syncing between different cloud mail accounts possible. Thus, it is important that, once you've enabled message-tracking, you should always enable it in subsequent requests.
• When syncing after the initial fetching,
  • The newly downloaded messages will be appended to the .mbox file, thus the off-line mail storage will have the exact same copy of the online version.
  • However, if for the purpose of syncing messages between different cloud mail accounts, then we don't want the old messages to get in the way. I.e., we want the final .mbox file contains nothing but the newly downloaded messages, so that when sending each message from within the mail box file as separated email to another (cloud) mail account using formail, the old messages which have already been sent, won't be sent again.
    To do that, execute the following steps so that the the final .mbox file contains nothing but the newly downloaded messages: 0. Make a copy of the off-line mail storage, both the .mbox and the .yaml file 0. Empty/truncate the .mbox file (e.g., "> my-label.mbox") 0. Start syncing after the above step (e.g., "cloudmail sync -f my-label -t ...") 0. Optionally,
    • Send out the newly downloaded messages with formail
    • Restore the off-line mail storage
    • Redo the syncing again, so that the off-line mail storage will have the exact same copy of the online version. You actually don't need to redo the syncing again, if you can concatenate the new .mbox file to the end of the old backed up .mbox file. You can do it if you know what you are doing, but I'll just simply say redo the syncing again.

To access Gmail via IMAP, you need to allow less secure apps to access your account.