Please help. Reviewer invitation mails blocked

We are using OJS We are having great difficulties in these days as we have troubles to reach reviewers that are essential for the editorial processes of a journal.

I see that most of our review invitations are not received by the potential reviewers. Sometimes I get warning emails that tells that the email is blocked. Sometimes I get no info but I see that the mail is not received.

One of the mail example that tells it is blocked is given below: host[]
said: 550-5.7.1 This message does not have authentication information or
fails to pass 550-5.7.1 authentication checks. To best protect our users
from spam, the 550-5.7.1 message has been blocked. Please visit 550-5.7.1
Prevent mail to Gmail users from being blocked or sent to spam - Gmail Help for more 550
5.7.1 information. 9si369980yby.37 - gsmtp (in reply to end of DATA

Example is from gmail but I see that some other institutions’ servers also blocks our mails.

I have tried to use another way to send mail by using “Use SMTP for sending mail instead of mail()” option given in OJS. However, I have failed to do so. I give some of the error_log file content when I failed to sent emails below. I did not understand anything from it but it may help ones with more knowledge.

Please someone help me to get this problem over. I have to prevent our emails to be blocked when using the default way with OJS or figure out how to send emails with “Use SMTP for sending mail instead of mail()”. May be both.

when I dig the matter more I see that the relevant part in error log is:
[01-Mar-2019 16:37:10 Europe/Istanbul] SMTP connect() failed. when I tried to use SMTP. I examined many things on the net and see that it is beyond my understanding. I do not know how to solve this issue.

I have searched many pages in net and in this forum. Somebody seems to solve their problem but I cannot retreace it as there is not deatiled information on how to do.

I hope someone can guide me.

Thanks to the contributors of this great Project by the way.

Error_log is deleted as it looks mostly irrelevant.