Defamation is unlikely to be an issue
If they wrote it it’s true and truth is a defense to defamation.
However, if you selectively quote out of context to portray the person in a harsh light, that can be defamation. For a contrived example, quoting only “The policies of the Nazis were just” and leaving off “... a symptom of the abhorrent people in that organization” is clearly defamatory.
Similarly, attributing motives to the author beyond what the plain text says can also be defamation.
There are still a number of other issues:
- Copyright - you don’t own this so, prima facie you cant reproduce it unless if falls within a fair use/dealing. Given your usage this is certainly arguable but not certain.
- Expectation of privacy - depending on the nature of the correspondence the person who sent it may have a reasonable expectation of privacy. While this is unlikely to apply to official correspondence of an organization, it may apply if the correspondence is more personal or intimate.
Why identify people or organizations?
If your purpose is, as you say, to highlight your experience, why do you need to identify specific people or organizations? Anonymous or broad examples (“an airline replied” etc.) would be just as illuminating.
Do your research
You use the expression “non-compliance”. Be precise with what is not being complied with, whether compliance is mandatory or voluntary and what exemptions exist. Usually, compliance requires “reasonable efforts” or similar - trivially is is not reasonable to exclude nuts from a peanut butter factory.