I had a couple issues moving my messages over to Gmail, so for my own benefit (and perhaps for others), here are the steps I took to migrate all my Notes emails to Gmail.

There might be better/easier tools, but I used Thunderbird to perform the migration. I configured IMAP accounts in Thunderbird for both Notes and Gmail. Then I simply dragged and dropped messages from Notes to Gmail. Well, “simply” might not be the right word, but that was the basic premise. So here are the steps:

1. Create a rule in Notes to forward all messages to the Gmail account. I set this rule to run after the Junk Mail filter so that spam to my Gmail account is minimised.
2. Configure an account in Thunderbird to access Notes via IMAP. The Notes IMAP server is the server name displayed at the top of the left panel in Mail.
3. Configure an account in Thunderbird to access Gmail via IMAP. I already had a Gmail account with IMAP enabled, so I simply followed the instructions in Gmail help to configure Thunderbird.
4. Recreate the Notes folder tree in Gmail. Each Gmail folder becomes a Label in Gmail. There are some odd implications of this mapping, so read the Gmail IMAP help to understand this.
5. Copy the messages from each Notes folder to the corresponding Gmail folder by drag and drop while holding the Control key down. This is where I started having some problems. It seems that there is some kind of rate-limiter on the Gmail side. I would get an error after about a hundred messages or so and the copying would stop. To work around this, I just selected two or three screenfuls of emails and copied the emails in small batches. Needless to say, this was a slow process and took me about two days.
6. Messages in Notes’ “Sent” view do not appear via IMAP, so I dragged them all to a folder in Notes so that I could access them via IMAP. Notes has a bug here. It doesn’t convert its internal addressing scheme (i.e. “Brent Muir/Adm/Staff/Monash”) to a valid email address in the “From” header line for sent mail. Thunderbird and Gmail don’t complain about this, but I didn’t like it as it looks broken. So to fix this, I first made a copy of my sent mail to a local Thunderbird folder. I closed Thunderbird and opened the mailbox file in a text editor and performed a simple search-and-replace to fix the “From” line. I restarted Thunderbird and rebuilt the index on that folder. Finally, I copied the messages from the local folder to the “Sent Mail” on Gmail.

Of course, I could have stopped after step 1 if I didn’t care about my previous emails. But I do care and I’ve got a lot of messages that I want easy access to. Gmail’s search ability is a compelling reason to migrate my mail.

Note that there are some quirks with this process:

• In Notes, all messages now have a forwarded status icon, but unlike normal forwards in Notes, the header doesn’t say where it’s been forwarded to.
• Most formatting (font colours, bullets, numbers, etc.) of Notes email is lost (they appear as plain text). This seems to be an artefact of the Notes IMAP implementation. Strangely, email that is sent from the Notes client to non-Notes users appear to preserve the formatting, but mail forwarded via a rule does not preserve formatting.
• Message counts between Notes and Gmail probably won’t match exactly. I initially thought that something was breaking in the copy process, but when I had a closer look, I realised Gmail automatically collapses duplicate messages into a single message. (We’ve got a couple team mailing lists and when I send to one, I get a copy sent back to me, so I had quite a few duplicate messages.) Also, because Labels are applied to an entire conversation, not just individual messages, some of my messages would appear in Gmail folders other than the one I had copied the message to.