SMTP madness -> solved!
Over a year ago I had all this with my ISP - they provided SMTP but it was restricted. I use 4 email addresses across 2 domains and sometimes need to send a fairly small mailshot to about 1500 people (perhaps once a month).
My ISP would only let me send from the 'free' email address they provided = no good. My email host limited the number of emails per hour I could send etc. Do not want to use Google as although they are free have concerns over privacy and think they also limit xx messages per day.
After a bit of searching I found http://www.authsmtp.com - monthly message quotas, sending from multiple email addresses, virus scanning and more - been using it ever since from my home and work connections, 3G when mobile and abroad - no problems - just as it should be.
It's not 'free' but for £1-2 a month it pays for itself 10x over at least.