I have Gmail set up in the default email client on my n810 using POP. The problem is when I write an email on my desktop and send it. The n810 gets it and thinks it's an incoming mail. How can I fix this?

Strikes me as odd that nobody else has ever encountered this problem? or maybe they just have no fix for it?

Hmm.

Hey killdeer.....the same thing occurs for me as well. I don't think that there is any way to "fix" this. Your e-mail client on the N810 is both retrieving mail from and sending e-mail through your Gmail account, so any/all mail would be captured by the e-mail client. The same thing also happens to me when I'm using Evolution (on Ubuntu). I'm not aware of any way to change this. Hope this helps.

Isn't this about the "All" folder which is a default virtual folder in GMail that includes ALL sent an received mail? Or does this pop up in the Inbox itself in Modest?

Modest seems to download EVERYTHING. Because the sent emails show up directly in the inbox. There is no "All Mail" folder on modest.

On my N800, Modest saves my gmail to my external SD card, how can I change it so that it saves to my internal card?

If you're using Gmail, don't use POP - use IMAP. It's much better at handling these sorts of issues, and Google support it directly now.

i am at work and not able to access gmail on my desktop but i believe that there is a setting that says deliver a copy of all sent email to my inbox. i suspect you have that checked off
but do as @jaffa says imap way better mail protocol

Thanks guys I'll try that out. Everything I found in my search said that people were having problems with Gmail and IMAP. I guess that's been fixed since

IMAP is the way to go anytime you can.