we test emails by sending them ourselves, at work addresses (outlook) and then to other addresses, so we can view on desktop, phones and tablets. and with display issues and rewrites, we tend to test a lot, it seems like no one can code an email the first time...it's a common problem with all of the different platforms we have to perform on.
but with all of that testing, I stumbled onto a few quirks with gmail.
the first is caused by the way gmail strings together the conversation, what I have found is for lack of better understanding, gmail caches the message. the actual content may change, but any images that I changed out or optimized in later messages continued to display the original image. i admit when we are that close to deployment, i don't change the file names of the images, so google just caches them instead of pinging the server again. to remedy this, i now delete any previous messages before I send a test message again. this "trick" took me a while to figure out, i thought I was going crazy and that my image server was broken and that I was uploading the wrong code to the esp.
the second quirk is with gmail tabs. the first few tests went to the inbox/primary tab every time. at about test 10 (I'm not kidding - the above mentioned quirk was killing me!) the messages started going to the promotions tab. nothing changed - not the from address, not the subject line, nothing except some images that we were optimizing. nothing we can really do about this, but it was interesting to see in action that gmail started to actively "promo" these communications because I hit some untold threshold...sadly, there is no conclusive data that the real send entered the promo tab instead of the inbox, or if my multiple tests had an impact on their placement.
so remember, if you have multiple test sends, and your looking for changes and not seeing them,delete them all - including emptying your trash - and that you're not going crazy...and when you can't find your tests after multiple sends, check the promotions tab.