i) Keep your mailing lists as clean as possible
Evacuate all invalid and withdrew email addresses from your mailing list. Sending to these emails tends to will make many bounce backs and will bring about delayed in email conveyance. Over the long haul, it will prompt your IPs getting blacklisted as well.
ii) Size of email
Maintain your email as straightforward as possible. Good content of email requires no attachments. If you have pictures and/or attachments, that is. It is recommended that you first host them externally and then grant a hyperlink to the recipient.
iii) Schedule your email campaign
Make sure you handle your sending time correctly when sending large amounts of emails. You should plan your next campaign for your email 1-3 days after the first email campaign. This ensures that all your messages are sent and all emails rebound collected.
iv) Internet connection
Check your home or office internet speed as it also affects your delivery speed.