[X-Unix] Expression/script edit series of txt files in separate subdirectories

[Andrew Kappy]

Any advice greatly appreciated.

Situation: directory with a long list of named sub-directories each  
containing a text file names "mailto". Each mailto file is a list of  
email recipients separated by returns.

I've been trying to construct expressions (or shell scripts) that  
would go through all of the mailto files and change:
1) find all cases of "x" and change to "y"
2) add a line with "x" to all
3) remove "x" from all files

Thanks in advance to anyone who cares to offer any suggestions.

Thanks,
Andy