I hope this does not seem obvious, but when you use the PC at your in-laws, do they have Call Line Identification protected (CLID) ?
You know, if they phone somebody, and that somebody tries to see who rang them by dialing 1471, the voice says, "....the caller withheld their number.."
The ISP/modem must know who is using the line, so if they have a blocker on it, you must remove the block.
Enter 1470 before the ISP's dial up number.
You jump the Dial Code box, and put the whole number in the "telephone number" box.
I hope I've not given you a red-herring here !
I assume you can get into OE5 but on trying to send an email, it gives the error message and won't play.