The diplomatic answer
when someone says "you must think I'm an idiot" is to tell them how there's no way you could do whatever their job is (so long as it's not IT). I like to make the point that my job is IT, so I am expected to know this stuff whereas their job is something they should know all about.
I'm also reminded of a couple of stories:
Over 20 years ago now, I was doing an on-site install and training session for a customer. I was half way through the install when I was called back to the office (for what turned out to be nothing more than my colleague not wanting to answer the phone all afternoon by himself). Customer was not happy with me leaving, cue much grovelling and apologies etc. When I went back the following week to complete the install, I was met with a rather frosty reception. Right up to the point I started the training part of the process where I quickly discovered that my customer had never used a mouse before. She was so grateful that I patiently showed her how it worked and made sure she was familiar with using the new software that the complaint was forgotten.
I've also seen real email disasters - I was working for an organisation that had a) outsourced its IT and b) had some users with a lot of archival email going back many years. Due to mailbox quotas, these users had their archive on their local drive, which was bad enough, but one evening a helpful admin decided to run a script that would search local drives and remove any PST files from them, because local PST files were "not allowed".
The next day I had 6 or 7 PCs sitting in my test suite running data recovery on them. The users eventually got all their emails back and all their attachments, sadly none of the attachments were connected to the original emails and they were all called <generic filename xxx>.doc.