Recently I have migrated my old Wordpress blog to hashnode. Around 180+ posts have been migrated.
I never knew during the migration newsletter will be sent automatically. Right now around 180+ newsletters are sent.
If such migrations are happening often to platform I am worried about the deliver-ability of mails. I would strictly suggest avoid sending newsletter during migration or send as single newsletter (saw its coming soon)

Anand Vaidyanathan
Product Manager @ Hashnode
The development is in progress. Thanks for reporting this! We will update here, once it is resolved