First, you have an ancient version of AcyMailing, so I would strongly advise doing an upgrade to the latest version, which is 5.10.25
Second, you didn't mention which sending method are you using
And finally, did you configure DKIM, SPF and DMARC records in your DNS, which would make sure that you can present yourself to the receiving email server as valid source of emails and not someone who is spamming around?