I have exactly the same issue ever since all the emails were migrated to the messaging company.   I also use thunderbird and have heard nearly every possible excuse from them about these problems. I have even been told that they dont support Thunderbird.   Also I have severe problems with often having my IInet IP blocked from sending any mail as it gets blocked by Abusix, the messaging company tell me they dont provide IP so it is not their probelm, IIbnet tell me they dont provide email so it is not their problem.   My experience is that the tech support  of the messaging company is that they have no technical knowledge, they cant even understand that sending from my computer using thunderbird is from my iinet isp but if i log in using their webmail it sends from their server ip.   I have no idea how to deal with these unqualified people, they dont even know about their own IP and mx records are being changed which disrupts all forwarded mail from any domains.   Paying for a business NBN account to IInet but the messaging company have no idea what that means. ... View more