I can receive emails BUT can't send

It appears you're experiencing a common issue with AWS WorkMail where you can receive emails but can't send them. This problem is typically related to permissions between WorkMail and Amazon SES (Simple Email Service). To resolve this, you need to grant WorkMail the necessary permissions to send emails on your behalf. Here's how you can fix it:

1. Go to the AWS Console and navigate to Amazon SES.
2. In the left sidebar, find and click on "Verified Identities" under the Configuration section.
3. Locate the identity (domain or email address) you're using to send emails from WorkMail and click on its name.
4. Click on the "Create Policy" button.
5. Select "Custom Policy" and use a policy similar to the following, making sure to replace the placeholders with your specific information:

{ "Version": "2012-10-17", "Statement": [ { "Sid": "AuthorizeWorkMail", "Effect": "Allow", "Principal": { "Service": "workmail.REGION.amazonaws.com" }, "Action": [ "ses:*" ], "Resource": "arn:aws:ses:REGION:AWS_ACCOUNT_ID:identity/WORKMAIL-DOMAIN-NAME", "Condition": { "ArnEquals": { "aws:SourceArn": "arn:aws:workmail:REGION:AWS_ACCOUNT_ID:organization/WORKMAIL_ORGANIZATION_ID" } } } ] }

Replace REGION, AWS_ACCOUNT_ID, WORKMAIL-DOMAIN-NAME, and WORKMAIL_ORGANIZATION_ID with your specific values.

After applying this policy, WorkMail should have the necessary permissions to send emails on your behalf. If you continue to experience issues, you may need to re-add your domain in the WorkMail console, which can trigger a workflow to check and correct various domain settings.
Sources
Sending Email failed in Workmail | AWS re:Post