In my company we are moving from http to https for as many sites as we can. In the end we redirect from http to https with a "301 Moved Permanently" (and set the HSTS header).
Now following question came up: What happens if we send a HTML newsletter with references to images via http (e.g. http://example.com/image.jpg) after we set up the redirect?
Do "common mail clients" like Thunderbird, Apple Mail, Outlook (2010, 2013, 2016) etc. understand the 301 redirect and load the images via https (the redirect for http://example.com/image.jpg goes to https://example.com/image.jpg)?
If not: What do they do instead?