can't send email, only receive

Same with three email prog.s (OE, Pegasus, and Incredimail). Settings for
ISP are correct (dialup access). Messages are something like host not
responding. (Similar settings on other PCs with same ISP - fine, so no
problem there.)

Receiving is OK, and obviously MoDem is sending/receiving at the lower
level: can log in to (and get recognised/acknowledged by) ISP, receive
email, and browse the web.

I presume some system file is corrupt (or, just possibly, something wrong in
the settings, though the DUN connectoid seems to be as it should).

Any idea which file to suspect (or setting to check)?

ISP (a UK one) is orange. (Previously known as Wanadoo, and before that
FreeServe. I know old identities are still honoured; although the SMTP/POP
servers have new names, I think the old ones still work too, though I've set
the new ones - I think they all point to the same two machines anyway.)