Hi,
i could not see in your posts how you exactly send you mails, but i suppose you have your own linux mailserver which handles the mails. If this is right you could have a look in the logfiles of you mailserver. At linux for sending mails often postfix is used. At my debian linux servers it writes all communication to the logfile /var/log/mail.log
If one of my dedicated linux server for a customer with mautic gets blacklisted from a provider like t-online in germany, i will see it in the postfix logfile like:
Jun 28 00:56:54 skom postfix/smtp[13413]: 3D6D440AE3: host mx02.t-online.de[123.123.123.123] refused to talk to me: 554 IP=111.111.111.111 - A problem occurred. (Ask your postmaster for help or to contact tosa@rx.t-online.de to clarify.) (BL)
There could be other error codes like “550"”, “5.7.1” and often you will the the email-adress to which sendmail ist not able to send mails, because of the blocking.
Like in my example with t-online you can send a mail to the abuse adress of the provider and ask for a delisting of your ip. But you have to explain why you think its really ok to do it 
In germany for example for t-online normally get an answer with 1-2 hours and after 2 hours the ip is delisted.
Ofen you also will see a link to a webpage in the logfile where the provider explains how you can get more infos and how you can ask for delisting.
The main reason why my linux servers with mautic got blocked at the beginning was, that the “reverse DNS/PTR entry” for the server-ip did not points at the same (sub-)domain i used for email sending. For example if the server IP is 123.123.123.123 and i use info@companyxyz.com for sending mails with mautic, the reverse DNS/PTR entry" for 123.123.123.123 should be for example mail.companyxyz.com.
You can check the reverse DNS/PTR entry for a ip also with https://mxtoolbox.com/SuperTool.aspx
I also set the “HELO/EHLO” for postfix to “mail.companyxyz.com”.
What you also should use is SPF, DKIM and DMARC in the dns settings of your domain. This way you also show the mailproviders, that you do a lot to prevent that your server/domain is used for spam.
And if you use a new IP adress for sending mails, you should “warm up” your IP. Means that you slowly increase your mail sending volume from day to day and that you should not start directly with sending a newsletter to some thousand at the first day.
I also configured my sendmail mailserver, that the amount of mails it sends to the same provider-domain (like @gmail.com) within a short period is limited. For example if my customer sends out a newsletter to 1000 receivers and if 200 are @gmail.com adresses, the mailserver will wait some seconds, after it sends the next mail to a @gmail.com adress. But the limit is only per domain, so only the mails to the same domain will be delayed a little bit. Id did this because if my sendmail is sending to fast, i also saw errors in the sendmail-logfile that different providers told the server “do not send so fast”
And of course its good, if you use the “bounce” function in mautic, so that it removed/blocks mailadress which bounced, because as i know different mailproviders also do not like it if you send mails again and again to the same adresses which for example do not exist anymore.
You can have a look at https://support.google.com/a/answer/81126?hl=en to see for example what gmail likes and you also can use https://support.google.com/mail/answer/6227174 to get mor infos what google thinks about the ip of you server.
Of course if you use email sending providers like amazon ses, sendgrid … then you do not have to setup all the things i wrote about, but then you have to pay for each mail.
I hope my post helps you or other people, who want to use their own mailservers
Matthias