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As dsefvonz has noted, you don't necessarily have to use whonix on the workstation VM but you better be confident in your ability to secure that VM if you do choose to go this route. Using whonix on both VM's is a lot more fool proof.
If they are blocking those ports I would be curious to know what they aren't blocking. As for getting around it, I'm not sure how you'd go about doing that or if you'd want to (possible ISP trouble). I'd maybe try posting a question about the port blocking problem specifically in another stackexchange forum. Sorry I couldn't be of more help.
If I'm understanding their website correctly you'd want to be using port 587 if you wanted to use STARTTLS. So either you should change from STARTTLS to SSL or change from port 465 to 587 for SMTP. I'd recommend switching from STARTTLS to SSL.
