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Got this email from FreshInbox —
"It looks like Gmail Classic is currently stripping ids and classes from elements within emails.
Gmail rolled out a new version of the UI in beta in April, however it looks like in the process of migrating users to the new UI a few days ago the "classic" Gmail suddenly reverted to a pre-2016 version before ids and classes were supported. This article highlights the changes in the 2016 update.
This means that your email might render differently in the old and new Gmail especially if you're using embedded styles (CSS in the <style> blocks).
If you've stopped inlining your CSS, you may have to consider inling your CSS again - at least temporarily."