Hello,
There is no simple answer here and usually you should get the reason for message rejection from the postmaster / support team of the remote email service that didn’t accept your message.
Now, because receiving an answer from them it may take time the usual recommendations are:
1/ check your IP used to send external messages into the DNSBL lists - for example here
2.1/ check if your IP used to send external messages have a reverse DNS entry
2.2/ check that that reverse DNS entry is the same with the hostname used to present (into EHLO command) to remote email services
3/ configure SPF for your domain and check that all IP addresses used to send external messages in the name of that domain are included into the allowed list
4/ configure and implement DKIM
5/ configure DMARC for your domain
Now, if all of the above have been checked you may use some online tools that may explain what is not working fine, like:
HTH,
Ioan