Thanks for response. The problem started when our nbn stopped working after a lightning storm. Instead of reviving it, because I was away studying, my husband bought a Mobile Wireless Broadband device from Telstra which got him back online but caused a problem sending emails, which sounds like the situation you have described. I however, because I didn't want to lose my dodo email address just yet, bought a dodo sim card and an unlocked device, and started up a dodo mobile wireless broadband account. Because it was a dodo sim card and account I thought the email sending problem would be over for me but not so. Dodo techs couldn't explain it and said I should ask the email client.

I haven't been using a phone for Internet - it's a device that calls itself a 4G ZTE WiFi Modem. It was sold as an Optus prepaid but Optus told me that they didn't think it was locked and as I was having no trouble getting on the Internet I thought it was all systems go. But maybe it's causing the problem. I'll try your gmail solution - didn't know gmail would do that. If worst comes to worst could buy a generic-type device. Thank you very much for your help.