What are "Client letters and/or advices"? Are these emails or printed letters? What do you want to know about testing them? Testing email systems? Spellcheckers? Please tell us what you already know and are doing. We'll fill in the gaps or offer suggestions for changes. It's very difficult to answer such a short question that doesn't tell us anything.
Is the data that varies from letter to letter correct? Is the format correct? Are there multiple pages? Is page numbering used and if so are the numbers correct? Is there bar-coding on either or both the letter and/or envelope? Is it correct? Is any text occluded? Is a watermark printed or on the stock already? If printed during the process, is it correct? Does the template meet legal requirements? Are multiple templates used? If so, is the right template used at the right time? Is the envelope bundled in the letter production? If so, is it addressed correctly? Is the letter metered as part of the same process? Is it correct? What happens if the output device fails in the midst of production? How does it recover? Does it recover correctly? If the letters are queued, is it possible that queue capacity could cause some letters to be dropped? Are the letters printed in different languages? Are the translations correct? Are letters archived? Is imagery involved? Are logos involved and correct? The list is vast as you can see! And, it goes beyond what is suggested here.
Of course if this is spam or junk mail - I believe your tests need to prove that all letters need to be addressed to the sender.
No spamming here, just looking to improve the quality of an annual mailout to the business' existing customers.
Data input comes from a number of applications including, a customer details database and three separate applications (Financial industry).
Data integrity issues are currently being addressed within the feeder systems.
Most documentation I've seen does not directly address this type of testing at the practical level. I'm looking to ensure the business testers have the best set of guidelines I can put together to help in the production of their test cases.
The suggestions posted by JakeBrake is the information I'm interested in.