I have come across Greylisting that regards all UN-official IP addresses as spam so bouncing the email,
That's actually not greylisting. Greylisting is simply deferring unknown senders for a defined retry period or number of retries. This is because a large percentage of spam servers don't ever retry. It has _nothing_ to do with dynamic IP's.
Additionally, servers that don't accept plain text SMTP are not SMTP servers. Encryption is optional. I run several mail servers, and I've _never_ had an issue with plain text SMTP. What I _have_ had an issue with is sending from an IP address without a valid reverse dns entry (and rightly so). In one case I maintain a cheap VPS with an SMTP server acting as a relay that I can bounce mail through so there is a valid reverse.
Most ISP mail servers will rightly block an outbound mail that is not from one of their own domains, so you can't send brad@gmail.com through the outbound mail server for wasp.net.au as it want's to see brad@wasp.net.au. Again, nothing to do with the SMTP protocol but policy on behalf of the server owner.
Sending SMTP from an embedded machine is not more difficult that sending it from anywhere else, your problems stem from trying to do things that most sane mail server admins rightfully block as spam (ie, not retrying when greylisted, sending from a dynamic IP where the reverse does not match any of the domains in the MX record or DNS entry). Again, I've _never_ struck a server that did not accept plain text SMTP. It *might* do that if you are trying to relay through it, but as a destination.. no.