If I sync my email using imap, then I delete the email on the web without deleting it on the device, can I opened email including images in the existing email on the device?
As long as the email has been downloaded before you deleted it and you since have not synchronised, then yes.
If it is an important mail, put your device in aeroplane mode and forward it to yourself. Then disable aeroplane mode and synchronise.
If the deletion has been synchronised, the mail is most likely gone. Unless, you have a trash bin which hasn’t been emptied…
Thanks for the response. I hope that when K9 becomes Thunderbird it will have a local folder like in the desktop version so that you don’t have to bother saving emails.