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joelw135…this does seem to be more common than I first thought…I spoke to Apple and my ISP about this issue but have concluded that it is an Avast issue. See my post here. I have found that switching off both the IPv6 and the other option in the Mail Sheild advanced tab is the only way I can get it to allow the delivery of emails. Very frustrating and if it starts to cause the same issue again in the future, Avast will be removed off my Macs.