erellsworth

Just use Contact Form 7’s normal shortcodes like so:
‘[email* your-email]’ 
‘[checkbox sendinblue-opt-in default:1 “Subscribe” ]’ 
‘[hidden sendinblue-list-id “13”]’
That’s all there is to it.

Contact Form 7 SendInBlue Opt-in Checkbox

Not working

It integrates the form with Send In Blue very well. =)

but if we are using the official SendinBlue plugin, they seem to conflict and the officil plugin stops working.

It is a very useful plugin.

WordPress plugin to add a SendinBlue Opt-in checkbox for Contact Form 7

InterServer

WordPress Mail.Baby SMTP Plugin (Simple E-Mail Delivery)
if you have problem with your WordPress site not sending emails? Then you got right plugin.
WP Mail.Baby plugin goal is to make email deliverability easy and reliable. We want to ensure your emails reach the inbox.
WP Mail.Baby fixes your email deliverability with various email smtp’s like Mail.baby, SMTP.com, Gmail, SendGrid, Mailgun, Sendinblue and more Api’s.
<h4>What is SMTP?</h4>
SMTP (Simple Mail Transfer Protocol) is an industry standard for sending emails. Proper SMTP configuration helps increase email deliverability by using authentication.
Popular email clients like Gmail, Yahoo, Office 365, and Zoho are in a constant battle with email spammers. One of the things they look at is if an email is originating from the location it claims to be originating from.
If the proper authentication isn’t there, then emails either go in the SPAM folder or worst, don’t get delivered at all.
This is a problem for a lot of WordPress sites because by default, WordPress uses the PHP mail function to send emails generated by WordPress or any contact form plugin like <a href="https://mail.baby/" rel="friend nofollow ugc">Mail.Baby</a>.
The issue is that most WordPress hosting companies don’t have their servers properly configured for sending PHP emails.
The combination of two causes your WordPress emails to not get delivered.
<h4>How does Mail Baby SMTP work?</h4>
Mail.Baby SMTP plugin easily resolves email delivery problems by improving and changing how your WordPress site sends email. We reconfigure the <code>wp_mail()</code> function to either use proper SMTP host credentials or leverage a built-in SMTP mail provider.
When using one of our built-in SMTP mail provider integrations (recommended), emails are sent using the provider’s direct API. This means even if your web host is blocking SMTP ports, your emails still send successfully.
This helps you fix all WordPress not sending email issues.
Mail.Baby SMTP plugin includes many different SMTP setup options:
<ol>
<li>
Mail.Baby (Recommended)
MailBaby is an email smart host that offers outbound filtering. Emails are sent to MailBaby systems, and are analyzed for content.
Email is then routed through an email zone based on the email content, and score of the email, or bounced as spam. IP reputation is handled by MailBaby.
MailBaby monitors all our ips for blacklists, and works with email providers through feedback loops and other abuse monitoring to ensure email delivery.
</li>
<li>
Sendinblue SMTP (Recommended)
</li>
<li>
Mailgun SMTP
</li>
<li>
SendGrid SMTP
</li>
<li>
Gmail SMTP
</li>
<li>
SMTP.com (Recommended)
</li>
</ol>
For most options, you can specify the “from name” and “email address” for outgoing emails.
Instead of having users use different SMTP plugins and workflows for different SMTP providers, we decided to bring it all in one.

Mail Baby SMTP

Send email from your WordPress site using  Mail.baby, SMTP.com, Gmail, SendGrid, Mailgun, Sendinblue and more Api's and Configure wp_mail() with them.

Contact Form 7 SendInBlue Opt-in Checkbox vs Mail Baby SMTP