Hallo Wolfgang,
I have problems compiling this with ConTeXt ver: 2010.03.20 22:59 MKIV [...]
I can't reproduce this. Can you take a look at the end of the log for this information:
correspondence : t-correspondence.tex: 2010.02.28 correspondence : t-letter.tex: 2009.10.10 correspondence : default.nli: 2010.02.28 correspondence : dinb.nls: 2009.10.07 correspondence : default.nls: 2010.02.28 correspondence : label.nle: 2009.07.03 correspondence : addrentry.nle: 2008.12.26 correspondence : dina.nls: 2007.02.11
I forgot how to tell context to produce a verbose log, but the hint was right. I normaly install the letter module in a local tex tree, but have made an update before with call first-setup --extras="t-letter" and this has installed the letter module in the standard tree where it was not found. Sorry for this! Wolfgang