I would like to have the possibility to send mails on form submission from WIX directly without the need for Sendgrid. The free sendgrid account is unreliable.
This is why the free sendgrid account is unreliable. If you've got the free account from Sendgrid you're sharing an IP address with many other users from which sendgrid sends your email. If this IP address has been abused by another user you can end up being blacklisted. In my case I see that mails sometimes are blocked and sometimes not because the IP address is also changing. I've mailed sendgrid about it and their only solution is that you take a payed account because then you get your own IP address.
Conclusion: The sendgrid free account sucks since you do not have any guarantuee that your mails arrive at their destination.