There are a few reasons:

• Make sure you have added the SPF record correctly, without duplication: https://cloudwish.com/guides/which-records-should-i-add-to-make-my-email-working/
• Add DKIM record(Optional, but may improve deliverability): https://cloudwish.com/guides/how-to-set-dkim-record/
• Add DMARC record(Optional, but may improve deliverability): https://cloudwish.com/guides/which-records-should-i-add-to-make-my-email-working/
• Check you have inserted the records correctly with the following tool: https://mxtoolbox.com/SuperTool.aspx
• Your domain age is under 30 days.
  There is a new policy that Blocks NODs (Newly Observed Domains). The following rules are added to your domain by default(We can’t do anything)
  • SEM-FRESH — Domains registered in the last 5 days
  • SEM-FRESH10 — Domains registered in the last 10 days
  • SEM-FRESH15 — Domains registered in the last 15 days
  • SEM-FRESH30 — Domains registered in the last 30 days
  • DAY OLD BREAD — Domains registered in the last 5 days (requires Registration to use their DNSRBL)

  Solution: You have to wait for your domain to be removed from the SEM-FRESH list. It will be removed automatically.

• Check your account is healthy in MailWish and not suspended
• Make sure your domain is not blacklisted:
  Check here https://mxtoolbox.com/blacklists.aspx