[Action required] Emails not sent by sandbox SES to certain verified emails

Hi,

I am using AWS SES to send some emails from a Backend, running in AppRunner. Since the project is still under construction, the AWS SES is still using the sandbox. In order to debug the email sending feature, I have verified some emails accounts in AWS SES, since I need to check several use cases involving both admin and non-admin users. However, I have empirically checked that some of the verified email accounts never receive emails from AWS SES, whilst others do. Can you tell me if there is any restriction to the email accounts that can receive emails from AWS SES after being verified for using the AWS SES sandbox?

As additional information regarding the email accounts used, whilst some gmail accounts can normally receive emails from AWS SES other gmail accounts cannot? Is there a limit to the number of verified email accounts in AWS SES sandbox? Any other type of limitation that could be interfering with some emails?

Thanks in advance for any help.