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Whitelisting like Greg mentioned would be a great first step. You could also try creating a few of your Sales people as test leads in your instance and try sending an email to them through a flow action. Then check their lead record's Activity Log afterwards and see what happened--was the email delivered, bounced, soft bounced, etc.. Marketing Suspended is different than Email Suspended, but they frequently get confused. If it is Email Suspended, that happens when an email is bounced. Just one possibility, but the bounce messages usually carry extra details on why it was bounced--look for bounce Category.