Question : Problem: How prevent iPhone from deleting old emails?

How do I prevent my iPhone from deleting old emails?  I want to keep them until I MANUALLY delete them.

Answer : Problem: How prevent iPhone from deleting old emails?

They will not be deleted untill memory becomes full.
And iphone has a large memory (100's of mail will remain stored)
"Where is the auto-delete emails settings on the iPhone"
settings / email

Reset Settings:
Tap Settings > General > Reset > Reset All Settings
Note: This will reset all settings. No data or media will be deleted.
Tweak your iPhone settings to fit your needs:
http://searchmobilecomputing.techtarget.com/generic/0,295582,sid40_gci1289394,00.html
Video guide:
www.ehow.com/video_4445851_change-iphone-settings.html