Localstack Custom Email Sender Bug

Steps to reproduce

Run localstack: docker run --rm -it -p 4566:4566 -p 4571:4571 -e LOCALSTACK_API_KEY=api_key -e DEBUG=1 localstack/localstack
Install dependencies: npm run setup
Create Localstack stack: npm run cdk-local-stack
Run npm run test-bug to see the bug in console.

Name		Name	Last commit message	Last commit date
Latest commit History 3 Commits
bin		bin
lib		lib
test		test
.gitignore		.gitignore
.npmignore		.npmignore
README.md		README.md
bug.ts		bug.ts
cdk-exports-dev.json		cdk-exports-dev.json
cdk.json		cdk.json
jest.config.js		jest.config.js
package-lock.json		package-lock.json
package.json		package.json
tsconfig.json		tsconfig.json