"However" vs. "but" — which is more formal?

I realize there are questions on the correct usage of "but" and "however". In this case, I am concerned with correctness in a formal context.

I have heard it said that however should be used in formal mail communications instead of but because the word "but" has a negative connotation. Which should I use in, say, an email to a client?

Examples:

• We understand the issue you have pointed out, but we cannot consider it as a defect in the application since the current behavior was requested by your team.
• We understand the issue you have pointed out. However, we cannot consider it as a defect in the application since the current behavior was requested by your team.

Solution 1:

Both can be used in a formal context, but the question is not so much whether you use but or however, but whether you use one sentence or two. That to some extent depends on the style of the rest of the text, but it also depends on the extent to which you want to link failing to consider it as a defect to the understanding of the issue. The main point here is that there are still people who do not consider however to be a coordinator, so that if you do use however it is advisable to begin a new sentence with it, as in the example, and not to use it to join two clauses.

Solution 2:

But does not only have negative connotation in Indian interpretation. Think about its usage in arguments. It has an 'objecting' connotation; it can be used more aggressively than however. And strangely perhaps, it can also weaken your message.

I learnt that but can weaken a sentence in an assertiveness training course. We were taught to say something like "I understand that you want to go out with your friends, and you have have to do the washing up before you go". Of course this was about verbal communication, and it sounds stilted at first, however, there is a point* to the construction. I came to understand that this can apply to written work as well, which may be one reason why we are taught to avoid but in written work (this does not mean it cannot be used when it is the best word). Look at what you have written and see if but actually weakens what you are trying to express, even though it may seem like the right word. Sometimes you can replace the but with and, or another word or construction, and it works surprisingly well. * But sounds like you are arguing; and is a firm statement.