If you use a custom domain with a non-standard TLD for your email, have you experienced any issues like these before? Do you think it was the non-standard TLD's fault? I've seen people claim, for example, that using smaller email providers can also cause issues.
I know very little about how emails are flagged as spam, so I'd also appreciate any advice you'd give to someone who has only used a Big Email Provider for all their life and would like to now switch.