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.
The problem isn't tech savvy companies but the rest of them. I can't use my preferred email with my insurers. I actually had a kafkaesque situation with my health insurer (Kaiser) a few years ago where I was able to use the weird TLD initially and then they changed their front end email parser to reject it so I couldn't log in. Which sucked in the early covid days when I needed telemedicine for everything.
Setup your SPF and DKIM fields correctly and it will help email providers confirm you are who you say you are.