Streak's mail merge tool is a layer on top of the Gmail API, which means when you send a mail merge via Streak it's not a whole lot different than sending an email directly from your Gmail/G Suite account.
Because we send emails via Gmail, our recommendation is to follow Gmail's best practices when creating a mail merge. Gmail has a great article here overviewing their best practices.
You will increase the chances your email is delivered by only emailing people you've previously been in contact with, as well as people who've requested to receive your correspondence.
Another good rule of thumb is to write your mail merge content as a personalized, one-off message to the recipient. If your email contains generic, impersonal content, multiple links, pictures, and is written like an advertisement, it's more likely the recipient's email client will mark the email as spam or reject the email altogether.
Receiving Bounce Backs When Sending a Mail Merge
Streak doesn't send out any bounce back error messages, so if you've received them they're likely coming from your Gmail account and/or the recipient's email client. You can Google the specific error message inside the bounce back message to find the specific reason the email failed to reach the intended recipient. Some common reasons are:
The recipient's email address doesn't exist - This could be because there's a typo in the email address you're attempting to send to. Or, it could mean the email account was closed down.
Blocked - The recipient's email client is blocking your message from being delivered. This could be due to the email client having strict filters in place for delivering emails. If you've emailed the email domain before without permission, it's possible the IP address you're sending from has been blocked.
Mailbox Full - This could be because the recipient's email client has reached its maximum capacity and is no longer able to accept incoming email messages. It could also be because the email account is no longer actively used.
Undeliverable - This could be that the email client is temporarily down or cannot be found. The client's server also could've crashed or could be under maintenance.
A Gmail/Google restriction - If Google detects unusual activity from your Gmail account, such as sending a lot of undeliverable emails, it's possible Google may temporarily block you from sending more email message. This seems to be more likely from a brand new email account. If this is the case for you, you will need to contact Google Support.