AWS Cognito admin create user sometimes not sending email

I use the adminCreateUser to create an account for my user with a temp password sent by email. Here my lambda code :

const signup: ValidatedEventAPIGatewayProxyEvent<typeof schema> = async (event) => {
  const email = event.body.email.toLowerCase();
  const UserPoolId: string = process.env.COGNITO_USERPOOL_ID;
  const param: CognitoIdentityServiceProvider.Types.AdminCreateUserRequest = {
    UserPoolId,
    Username: email,
    DesiredDeliveryMediums: ['EMAIL'],
    UserAttributes: [{Name: 'email', Value: email}],
  };
  try {
    const result = await cognitoClient.adminCreateUser(param).promise();
    console.log({User: result.User, response: result.$response});
    return accepted();
  } catch (error) {
    console.error({error});
    return internalServerError(error.message);
  }
};

Cognito is configured to work with SES. Here is my Cloudformation configuration :

  CognitoUserPoolPerfectPostUserPool: {
    Type: 'AWS::Cognito::UserPool',
    Properties: {
      UserPoolName: "perfect-post-user-pool",
      UsernameAttributes: ['email'],
      AutoVerifiedAttributes: ['email'],
      AdminCreateUserConfig: {
        AllowAdminCreateUserOnly: true,
      },
      Schema: [
        {
          Name: 'email',
          AttributeDataType: 'String',
          Mutable: true,
          Required: true,
        },
      ],
      EmailConfiguration: {
        EmailSendingAccount: 'DEVELOPER',
        From: 'Perfect Post <contact@perfectpost.fr>',
        SourceArn: 'arn:aws:ses:eu-west-3:xxx:identity/perfectpost.fr'
      },
    },
  },

My account is out of Sandbox for SES. I have now 1000 users registered to Cognito and 160 of them never received the email with the code and are locked in FORCE_CHANGE_PASSWORD. My users tell me that the email is not in spam either, it is nowhere.

There is no error in Cloudwatch also in SES. My Bounce rate is low and I am not sending more than 14 emails per second, it's flagged as sent: SES dahsboard capture

I have made some spam check with websites like mail-tester.com or zerobounce.net there is no alert with my email content.

I am not seeing what I'm missing out. Or maybe it's normal to have bounce because my domain was registered in January and it's too young. Should I pay for a dedicated ip ?