I can't speak for outside the UK, but here the ISPs hand out non-static[1] IP addresses to residential customers. These IP ranges are in the spam-block lists that a lot of the other mail providers use. As such sending any email directly from your ISP given IP address is hit-and-miss affair. You don't even get any bounce notifications, the emails just disappear and never arrive at their destination, so you have no idea that something is wrong.
Some ISPs allow you to pay for a proper static IP to get around this problem, but again some of the ranges are still in the block lists. The only way I could guarantee that my IP wasn't blacklisted was to switch to a business account with my ISP and thus have IPs from a different range.
If you are stuck on a non-static IP, the easiest way around the problem is to send via the SMTP smarthost that your ISP hosts.
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[1] They are technically dynamic IPs, but are sticky in that you keep the same IP unless your router goes offline for more than a couple of hours.
Some ISPs allow you to pay for a proper static IP to get around this problem, but again some of the ranges are still in the block lists. The only way I could guarantee that my IP wasn't blacklisted was to switch to a business account with my ISP and thus have IPs from a different range.
If you are stuck on a non-static IP, the easiest way around the problem is to send via the SMTP smarthost that your ISP hosts.
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[1] They are technically dynamic IPs, but are sticky in that you keep the same IP unless your router goes offline for more than a couple of hours.