Oh hang on, hang on I get to use this:
"I think you misunderstand, Adam" - note the comma use 
OP - You're saying you want to do an SMTP check (ie connect to mailserver and verify the account exists) but then speak of regular expressions to validate that the string passed to you meets the RFC requirements? That's two things.
First up as Adam says, you can validate the email address using CFINPUT, and again server-side using isValid(). Incidentally isValid("email") is actually full of bugs if you read the email address RFCs, which sadly I had to once.
However if you want to actually test whether or not the address exists on the mailserver, no there's no way of doing this natively using CF. You could (if you were fairly adept at Java) write yourself some classes that look up the MX records for a domain and then telnet to it. Basic SMTP conversation would do you:
220 remoteserver.something.com Microsoft ESMTP MAIL Service
HELO yourdomain.com
250 remoteserver.something.com Hello
MAIL FROM:email@yourdomain.com
250 2.1.0 email@yourdomain.com....Sender OK
RCPT TO:theiremail@theirdomain.com
550 5.7.1 Unable to relay for theiremail@theirdomain.com
Unless you get a 250 in response to your RCPT TO command, it's an invalid account.
However.
There really is no need to go that far in order to validate account. Far simpler is to just send them a confirmation email with a link they have to click to verify their account.
Sending email to a non-existant email address will *not* get you onto spam blocklists. The only thing that will is if you send email to one of the honeytrap email accounts that people like Messagelabs set up. No-one particularly knows what these are (as that would defeat the point) but they're only ever hit if you just pick a domain name and mass-mail every email address prefix you can think of to hope some sticks. No-one is going to come to your website and enter one of these email addresses.
I suspect you're already doing enough to make sure you're not on the blocklists. Generally you only end up on one if your mailserver is sending spam, things are much more clever these days.
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