I had my Gmail account setup to be accessed via POP in Mozilla Thunderbird and thought it was about time to shift to using IMAP instead. In fact, it's that long ago that I can't actually remember why I didn't just use IMAP in the first place.

Why should you be bothered about how your email is accessed? As long as you can read it, what's the difference?

As explained in this Gmail blog post, when using POP, changes made on a local device (e.g. marking mail as read or sorting mail into folders) aren't reflected on the mail server. When you access your mail from a different device (or client), you'll find your mail organised as if you hadn't made any of those changes at all. IMAP, on the other hand, synchronises any changes you make locally to the mail server, so your mail will be organised in the same way when using IMAP, no matter which device or client you use. The changes will propagate to the next device you use.

With the prevalence of smartphones and mobile data connections becoming more usable, it's increasingly the case that multiple devices, rather than a single PC, are used to access mail. So, it seemed a good move to make the switch now.

The approach I used was the accepted answer here: setup the IMAP account in Thunderbird so that you have both the old POP account alongside a new IMAP account, and then just drag folders from the POP to the IMAP. It's not much more complicated than that, but these are the specific steps I used for migration:

  1. Enabled IMAP access on Gmail.
  2. Moved everything in the current Gmail inbox — all this mail was already on my PC via the POP account — into the bin. (I actually did this after I migrated all my folders, but you could do it here while logged into Gmail via the web.)
  3. Backed up my current Thunderbird profile, in case anything goes wrong or you want to quickly revert back to using POP for some reason.
  4. Setup the Gmail IMAP account in Thunderbird.
  5. Stopped the POP account from checking for new messages, to avoid the possibility of the POP account redownloading all the sorted mail that was currently being copied from the local POP folders back to Gmail. (Untick the two boxes in Tools menu > Account Settings > Server Settings > Check for new messages...)
  6. Dragged each folder from the POP account to the IMAP account. This will probably be by far the slowest step if you have plenty of mail. Don't forget your Sent Mail folder.
  7. Once I was happy that everything had migrated across, I emptied the Gmail bin, and then removed the POP account from Thunderbird (Tools menu > Account Settings > Account Actions > Remove Account).

Overall, it was simple. The only time consuming part was actually moving the messages from the copies I had in the Thunderbird POP account to the IMAP account.

The only other (optional) thing I did was keep a local copy of all my previous mail for quicker message loading and searching, and as a backup. Make sure that the appropriate account setting is ticked (Tools menu > Account Settings > Synchronisation & Storage > "Keep messages for this account on this computer"). Next, right click on a mail folder, go to the Synchronisation tab and click Download Now. Repeat for each mail folder.