delay in sending email by remote smtp server

I have two separate Linux servers. The first one is my web server and the 2nd one is a mail server. I have run mail server with Exim and SMTP. The problem is that when I want to send an email from my website through the mail server, it takes exactly 20 seconds for each mail to be sent! Here's the log for an email: 2018-12-03 18:49:23 SMTP connection from [Ipaddress]:40840 (TCP/IP connection count = 1) 2018-12-03 18:49:43 1gTsLn-0001aM-Hl <= Bounce@mydomainname H=static.ipaddress.clients.your-server.de (mydomainname) [ipaddress]:40840 P=esmtpsa X=TLSv1.2:ECDHE-RSA-AES128-GCM-SHA256:128 CV=no A=dovecot_login:info@mydomainname S=51605 id=MjA2MTEzOQAC394443Y57BAMTU0Mzg1OTM2Mzg@mydomainmame T="\357\243\277 Store - \330\250\330\247\330\262\333\214\330\247\330\250\333\214 \330\261\331\205\330\262 \330\271\330\250\331\210\330\261" for mytestaccount@gmail.com I also added the ip addresses on the hosts file of both servers and put IPs in the whitelist in firewalls. I don't have this problem on other servers in which web server and mail server are on the same machine. Any help is appreciated