Authentication
Mailbox providers check that your mail really comes from your domain. SPF, DKIM and DMARC are the three DNS records that prove it; Gmail and Yahoo expect them from bulk senders, along with a one-click unsubscribe.
Consent
Under section 69 of POPIA you may email someone for marketing only with their consent, or if they are an existing customer and the customer conditions are met. Sending to bought lists is the quickest way to the spam folder and to a complaint. Read the POPIA email marketing rules.
Segments and timing
Send each group what is relevant to it. Segments you define and automations that run on their own keep mail wanted, and wanted mail keeps reaching the inbox.