Because of the prevalence of spam, its becoming increasingly cumbersome to self-manage the delivery of transactional email. If you need registering users to click on links emailed to them, its a major frustration and drop off point when the emails don’t appear.
A recent aid to the email delivery toolset are services like Sendgrid.com.
Its a startup that for $80/month will take care of all those pesky SPF records etc and stop you having to email Yahoo! mail every 2 months when you get marked as a spammer by some goldfish brained new customer who thinks your registration confirmation is spam.
I faced many issues in this area last year and wish I knew about this service then.
Another consideration is using Social Sign On via Janrain or alternative providers to get round this. The only issue with Facebook login/OpenID etc is that there is no obvious way to enforce re-login for example when a customer goes to the My Account page.