Using conversion services like ZAMZAR it should be possible to convert EML files to TXT. However, you could simply copy and paste the content of opened *.eml files to a text editor and save it as a *.txt file.
Convert Email message to Simple text file.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert eml to txt on Windows
A flexible email viewer
Convert from eml file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.