Do not do this, using the IP that your companies mail server is running on (if that is the case at your company). Anything that bursts out so many mail messages at once, will cause that IP to flagged as a spammer. And it can take some time before you are allowed to send any mail again. It will also reduce the reputation of your own mail server, regardless.
So, if you have IP addresses to spare, use those. Seriously.
Seeing the screenshots from TurboMailer, they are from the Windows XP era. Back in those days it would have been less problematic to push out a lot of email messages at once. Nowadays, everything is much, much stricter and there are many organizations that patrol the internet to fight against spam.
*edit*
I forgot:
It often takes a day or 2 or 3 to restore your outgoing mail functionality. Perhaps one day before it is noticed that outgoing mail is not received and the intended receiver starting to complain, the you need to go to the site or sites that have you flagged as spammer to create a request for removal and than you need to wait until they pull you from their list. Sometimes this is immediately, with others you can wait for a day or 2.
And all of them keep your IP address in a sort of 'person of interest'-list for a few weeks. Do you burst out another email message this way in that penalty-period, you'll lose the ability to send mail for a week, if not more, depending on the guidelines of the anti-spam organization's guidelines. All of the organizations clearly warn you for this, the first time you'll make the request to be taken from their spammers list.
What you are ordered to do sounds more like a PR panic move than anything else. Not worth it to drag your mailserver's reputation through the mud for it, if you would ask me. But I know you didn't.
Maybe you could use a command line script to push out messages in a time-controlled manner.
In that case, I can recommend 'SwithMail' and 'CMail'. Then you only need to design a HTML message using HTML4 syntax, because that makes sure every mail client is showing the message as intended. Store it as a text-based file and use that file as the mail body in either command-line mailer. Then you only need a list of recipients and repeat sending the message to each of that list separately. Maybe that prevents you being flagged as spammer.
Or sending this message to yourself, with the intended recipient as a BCC. Don't know if that gets you flagged either. But perhaps worth a shot.
