How to send some emails without getting in trouble with ISP?

I want to send some websites an email telling them I would like to guest post on their site. My cocern is some people getting pissed off and reporting me to my ISP for spamming.

If there is a contact form on their website I can use a proxy to hide my IP when filling out the contact form and that should protect me.

but what about when there is an email address listed on their site? How do I send the email without getting in trouble with my ISP, because if they look at the headers, they can see my ip address. now you might be wondering, why don't you use a proxy to send the emails? the problem is, the proxy ip addresses are going to cause the emails to land in spam folder.

Get a mailchimp/sendinblue account and install https://wordpress.org/plugins/mailin/ on your wordpress site. Import the email addresses and send emails directly from your wordpress dashboard. The from IP address will have your hosting server's IP and not yours.

You can also do something similar with Interspire Email Marketer which I believe is shared in the https://www.blackhatworld.com/forums/member-downloads.29/ section.

Spam sometimes doesnt detected only because of content but also for newest ips that arent familiar and also if of mass email sent if you dont respect the waiting intervals and this time depent from isp to isp