The TP2 doesn't have as much storage memory as most newer devices, but it is plenty for most of us. If you've gotten that low memory error, you must've gone a very long time without a hard reset. Stop by the "CDMA Upgrades" subforum here and you'll be hard-resetting every couple of days.
Conversely, I would recommend the following steps if/when you do get the same problem:
1. Delete picturemail nonense. I cannot confirm that the stock rom stores them here, but I would delete everything under My Documents\UAContents. This is where all the sent and received pictures are stored as data files. I typically save the save-worthy pictures to my storage card, and have this directory wiped weekly.
2. Move personal non-default items from Internal Memory\My Documents to Storage Card\My Documents. This includes your pictures, notes, recordings, documents, wallpapers, etc.. Things like templates don't take much memory at all and you may as well leave them there.
3. For temporary relief, delete the HTC Album cache. On my phone, this is about 10MB of data. It is located in Application Data\HTC\HTC Album\cache
4. Go through your Program Files directories and see which application or game is storing unneccessary large files. Make a note of which programs you're not using and uninstall them if you are able.
5. Delete and re-create your email accounts. Before synchronizing, ensure that each account stores attachments to the storage card (in settings). As a sidenote, I'm not sure why you're unable to send/receive after the hard reset... maybe your ISP requires toggling SSL in order to connect.
6. Regular maintenance. Delete stuff manually, spend the time to make a custom script to delete things, or just use Clear Temp every once in a while:
Downloads - Windows Mobile Pocket PC - ClearTemp
As horndoctor said, I would seriously consider a different rom. A rom with the WM6.5 Start Menu removed and processor overclocked will make it like a totally different phone. If you're attached to the stock one, post up specifics about your email issue and we'll try to help.