fix json issue in mail parsing / writing

mail-service/pom.xml

@@ -31,6 +31,12 @@
 			<artifactId>spring-boot-starter-mail</artifactId>
 		</dependency>
 
+		<dependency>
+			<groupId>org.springframework.boot</groupId>
+			<artifactId>spring-boot-starter-json</artifactId>
+			<scope>compile</scope>
+		</dependency>
+
 		<dependency>
 			<groupId>org.springframework</groupId>
 			<artifactId>spring-web</artifactId>