When using the out-of-the-box SendGrid client provided by Bubble for getting started (rather than a personal email client with a dedicated IP address), SendGrid sends emails via a pool of shared IP addresses which may not be whitelisted or may be blocked by the security measures put in place by the recipients' emails. This means that while the emails are getting sent from your app successfully, they are not getting delivered.
In order to work around this behavior, there are three main approaches you can take:
Ask users with those impacted email clients to whitelist your Bubble app’s domain with their email account to ensure that they receive those emails.
Use an alternative third-party email service like Postmark or Mailchimp via plugins and custom API integrations.
Use a dedicated IP address from SendGrid once you have a custom domain for your Bubble app.