spring-integration-mail: RFC5322 conforming messages containing multiple values in `from` or `reply-to` headers cannot be transformed

[dl1ely]

In what version(s) of Spring Integration are you seeing this issue?

spring-integration-mail:5.4.5

Describe the bug

https://github.com/spring-projects/spring-integration/blob/main/spring-integration-mail/src/main/java/org/springframework/integration/mail/support/MailUtils.java#L52 and https://github.com/spring-projects/spring-integration/blob/main/spring-integration-mail/src/main/java/org/springframework/integration/mail/support/MailUtils.java#L56 expect a single address, and throw an exception org.springframework.messaging.MessagingException: conversion of MailMessage headers failed; nested exception is java.lang.IllegalStateException: expected a single value but received an Array in https://github.com/spring-projects/spring-integration/blob/main/spring-integration-mail/src/main/java/org/springframework/integration/mail/support/MailUtils.java#L69 while RFC 5322 https://tools.ietf.org/html/rfc5322#section-3.6.2 allows for a comma separated list of "mailbox specifications" (for from) or "addresses" (for reply-to). Messages that conform to RFC 5322 cannot be transformed successfully.

To Reproduce

This was just a find after an analysis of production log error messages when receiving mails. It should be reproducible with mail messages containing multiple values in the from or reply-to fields according to RFC 5322.

Expected behavior

from and reply-to header fields should support multiple comma-separated addresses, be extracted successfully and copied to the AbstractIntegrationMessageBuilder at https://github.com/spring-projects/spring-integration/blob/main/spring-integration-mail/src/main/java/org/springframework/integration/mail/transformer/AbstractMailMessageTransformer.java#L83

Inbound mail messages conforming to RFC 5322 should be transformable without throwing an exception. Currently, this leads to cases where we are not able to successfully receive RFC5322 conforming inbound mail.

[dl1ely]

Section 3.6.2 of RFC 5322 has been updated by RFC 6854 https://tools.ietf.org/html/rfc6854#section-2.1 but this does not change the problem described. Still from and reply-to may contain comma separated lists of addresses.

[artembilan]

Yes, we definitely need to follow an RFC, but it is hard to fix it without a breaking change since MailHeaders.FROM & MailHeaders.REPLY_TO were there for a while and definitely expected to be as a single value.

As a "soft" fix I suggest to convert an array of those values back to the comma-delimited single entry, so it won't be a breaking change for existing apps.

In the future major version we probably can revise it in favor of handy array as it is expected by RFC.

If this makes sense, I'll raise a PR with an appropriate back-port to supported versions.