Not true, or wasn't true I used to download it to my OE. I don't bother now I use their Web Mail which is easier now I'm on BB.
You obviously will be able to open messages you have saved in OE but will need to go online to download them.
OnePlus 5 review
Alice Saey's mesmerising animation for Dutch singer Mark Lotterman
iPad Pro 10.5in (2017) review
Comment booster votre iPhone ?