i had a few free email addresses, including a lot of gmails, & for about 2 weeks i just had meaningless conversations with the free email addresses, (talking to myself) what are you wearing today, what are you having for dinner, & when it started the emails sent were going in spam box with both yahoo & gmail & were taking ages to deliver, but after a couple of days both yahoo & gmail were putting them in "inbox" & were delivering them in about half a second, so that does work, the domain is about 3 years old, it is in a very spammy niche with an emd,
i then tried mail-tester & it got a 10 out of 10 so if you have free emails then just try talking to yourself for a while, just do 1 a day (whatever) & go back & forward