Even if a client or manager writes to express some concerns they have regarding your work, you can still thank them. This shows that you value their input and will take their concerns seriously. Alternatively, you may wish to use, “Thank you for your feedback.”
While thank yous at the beginning of an email are typically written to thank the reader for past actions, thank yous at the end of an email tend to imply you are thanking the reader for a future action. By showing your appreciation in advance, you are more likely to get a positive reaction.