"or if they redirected back to their own SMTP servers."
It was done that way back when it was Freeserve, and they gave the server that handled such mail quite low priority if it needed fixing. I've used external services, such as FastMail.net (which allows users to send on any port, with authentication, so can be used on a company LAN as port 80 is rarely blocked!).
Other than that benefit (from a paid-for service), I'd always suggest someone get themself a free mail address from anyone but their ISP
(whether it be GMail, Hotmail or some other) so they have the option of switching ISP without the headache of re-registering with websites where they have accounts.
Also, if you don't like junk mail, then using spamgourmet.com can 'hide' your real mail address from websites you register with, and if they ever change hands, or get hacked, you can quickly set spamgourmet to eat everything sent to that address. Also, if you use unique addresses, one per website, then if they sell their mail list at any point, you know exactly which website was the culprit.
If you run a business, have a second ISP and backup web hosting...
1996? Enterprise (IoM 0345) + Clara.net
1998? Clara.net + Freeserve
2000 Freeserve 500kbps (+ Clara dial-up + Clara USENET/Mail)
2002 ? Eclipse 500 kbps + FS
2003 ? UKFSN 1000 kbps + Eclipse
2005 ? PlusNet 8 Mbps+ Eclipse
moved home - used office BB
2008 Three dongle at home
2010 PlusNet 2 Mbps + Three dongle
2012 PN 10 Mbps + Three (ZTE 802.11 router) + Three mobile (802.11 hotspot)
2013 PN 30? Mbps + VM 30+? Mbps + Three mobile