Email deliverability and open rate are related to each other. Some of the elements of email deliverability of open rates are:
A hard bounce occurs when an email is sent to a wrong address and a soft bounce can happen when the message is rejected, etc.
The bounce rate should not be more than 5%.
Avoid third party server for sending emails.
Primary mailbox is always the best option to choose.
Keep the characters around 60.
Don't use capital letters and exclamation marks.
Avoid spammy words.
Use short subject lines, Example: 'quick question'
Tell the prospect, what is in it for them.
Time intervals between 1st emails and the 2nd
Send 25 to 100 emails per mailbox.
Use multiple mailboxes.
Country law: GDPR is also a very important part to acknowledge.
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