PHP Mail is the worse thing that you can use if you care about email delivery and you can test how good/bad is your email deliverability at dashboard/settings/tools
Also, please note that MS email (outlook, hotmail) will trend to flag as spam any message from a "cold" IP.
Gmail works but you have to use the right port and protocol (587 TLS). You can always rely on third-party services like Sendgrid or Sparkpost, both of them have a reasonable free tier.
Did you try that SMTP alone? Like actually use that SMTP outside Chevereto. Maybe your login credentials are wrong. If you are able to login to that SMTP account then the problem could be an openssl issue on your server.
If you are sure that the credentials are OK (works outside Chevereto) then you will have to send me server access, smtp and admin account credentials to check this.
Don't forget to include website url, server path and a link to this topic.
Most likely the issue is openssl or even a blocked port. In both cases, I won't be able to fix that and you will need to request assistance to your hosting company.