If these are being sent on newly warmed or un-warmed IPs then it is common for open rates to drop temporarily. Recipients would need to re-train their ISP filters to place these emails in the Inbox, add the new domain to their contact lists, etc. ISPs would need time to adjust their default filtering behavior as well. There could of course be factors involved that are not related to deliverability at all, such as lack of recognition from the users that the new domains are associated to previously solicited communications so they are simply failing to read/load images.
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