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Any help would be hugely appreciated.
Having been in this position myself I'd suggest that if you really want to cut down on bounces then you need to look at how you're handling the lifecycle of the data...
Are the errors coming in at the data collection stage?
If they are then you might want to consider improving the validation on your data capture so that you can address common issues on the spot. Bear in mind the only way to really guarantee a valid email address is to email it at point of capture and get the user to confirm it via a clicked link.
Are the errors only coming to light during the course of the mailing?
Normally this is a data age or data quality issue, and outside of actively re-confirming details semi-regularly the best you can hope for is to work through the errors and bounces so that you can determine which contacts were unable to be emailed.
Over time flagging bad data has a really positive impact on cleaning up an email list, making future maintainance much easier.
Of course it will lose addresses over time (people move ISPs, change jobs, mailboxes fill up, free email providers shut down etc.) but at least you're starting at the best possible point.
Beyond that, a search for "change of email address" on Google should help track down some of the companies that can provide list cleaning and verification services.
But as someone said there are software that can verfiy existance of domain or mail server. We use Groupmail with Email Inspector plugin for that purpose.