Same problem here - but I may have it licked!
I have just been wrestling with the same problem. I have several clients with bulk mail software based on PHP and MySQL, server-side, which works well - but one of those clients, after importing a list he has been using for a while sending only plain-text mail messages using different bulkmail to legitimate customers of his, had 200 bounces out of a list of 800 - he almost got his domain shut down!
Whilst the same list seems to suffer few bounces when sending plaintext only, the new mailer sends both HTML and a Plaintext version - it seems that HTML certainly raises the alarm!
I this morning downloaded software called MailLIst Validator Ver2 free of charge and used it to check his list - Find it at:
"Mail List Validator 2.1
Free Ultimate Mailing List Vallidator"
It seems to work well and reported about 160 invalid emails. I tried two others first but they were rubbish - could not get the SMTP/Proxy to work at all!
However I do not think it handles Hotmail accounts - putting in a fake hotmail address gave the report 'Host resolved" which is maybe technically correct (there is such a host!) but I suspect there is no address like 'chajam)email@example.com'!
But as I have already had major problems with numerous hotmail accounts on another mail list, I have simply removed them altogether and now do not accept signups from that domain.
At least now I reckon we can send to lists which are 'mostly' clean with a much lower bounce frequency which is a step in the right direction.
I am now looking for a php script which will run a check on all mails in the DB, using a cron, perhaps daily, and automatically deal with invalid mail addresses.