Document SMTP timeout settings

Closes gh-8213

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -4702,6 +4702,16 @@ namespace, see the
 {sc-spring-boot-autoconfigure}/mail/MailProperties.{sc-ext}[`MailProperties`] for more
 details.
 
+In particular, certain default timeout values are infinite and you may want to change that
+to avoid having a thread blocked by an unresponsive mail server:
+
+[source,properties,indent=0]
+----
+	spring.mail.properties.mail.smtp.connecttimeout=5000
+	spring.mail.properties.mail.smtp.timeout=3000
+	spring.mail.properties.mail.smtp.writetimeout=5000
+----
+
 
 
 [[boot-features-jta]]