Hi @Hassan, 

Welcome to the Community! Happy to give some more insight here! 

Did all the recipients who did not find the email check their ‘Spam’ and ‘Promotions’ tab of their inboxes for the email? Our Help Center has some insightful documentation on Why Some Preview Emails Go to Spam. Additionally, Klaviyo is only the sending service, not the delivery agent of emails. An inbox provider is the one that decides whether or not an email is received and if so, where it lands in the inbox. If these emails were sent to different inbox providers, the reason some inboxes didn’t receive the email could be because emails are being blocked due to an inbox provider's DMARC settings. 

Do you have a Dedicated Sending Domain (DNS) set up in your account? Essentially, inboxes block emails due to the misalignment between from-address domain and sending domain (Klaviyo). So without a Dedicated Sending Domain, the email is essentially saying it’s coming from one domain, but is really coming from Klaviyo's domain. Some inboxes may consider that a red flag and is why we recommend setting up a DNS, so your brand’s emails seem more trustworthy to inboxes!
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I’d check out these other posts made by users with similar issues to gain more insight! 

Thanks for participating in the Community!

-Taylor