AWS Cognito automatic emails are sometimes not been sent

Hello,

I'm using AWS Cognito as an authentication provider. I'm using AWS SES account to send cognito automatic emails.

When creating a user account, i send custom emails on my side (using AWS SES api), and cognito should automatically send welcome emails.

Some users told me that they received my custom emails but not emails with their credentials ... (event after checking on spam folder). I don't have dedicated IP address for my AWS SES account.

What would you recommend to me ? Go for a dedicated IP address ? (Will it really solve my problem ?) My application will only send emails when users are created (and users and imported by hundreds at same time), so that means lot of emails are sent at the same time. I set up a backoff algorithm on AWS SES API to avoid overloaded issues.

I already ask my AWS SES quota te be increased to 20 per seconds.

Thank you in advance.