Gmail says it requires the imported file to be in .csv format. I have
tried several variations, none of which has worked.
Since I am sending to a list containing only a number (about 50) of
addresses, I thought I could make a text file of the form

name1@place1.com
name2@place2.com

which doesn't have any commas in it, but the entries are separated by
line breaks. This was not acceptable to gmail.

So I tried

name1@place1.com,name2@place2.com

but got the same result.
Does the file extent matter? I tried file.txt as well as file.csv. No
success either way.

I haven't yet faced the problem of sending my message as BCC.

I'm running Windows XP, Home.