AcaCandy's reply 27-Apr-2004 10:57 PM struck a chord. Our internet cable provider is cablecom.ch, who use servers named hispeed.ch. My email via cablecom.ch works fine. My now-living-here girlfriend came with an account @datacomm.ch (now owned by tiscalinet.ch). We just had a disk failure, and had to reinstall stuff. Maybe the re-install was faulty, but since then she could receive but not send, getting exactly the "Port 25, Socket Error 10060, Error Number 0x800CCC0E" message.
Solution as given by AcaCandy: use (our) ISP's outgoing mail server, not (her) mail provider's server.
Tools > Accounts > Mail Properties > Servers > Outgoing mail (SMTP)
change from smtp.tiscalinet.ch to smtp.hispeed.ch
Many thanks!