If you receive the following bounce back some time after sending an email from your Exim/cPanel SMTP server :
all hosts for ‘recipientdomain.tld’ have been failing for a long time (and retry time not reached)
Using the “exinext” command, you can have a clear view of the issue :
[root@sendersmtpsrv log]# exinext recipientdomain.tld Transport: smtp1.recipientdomain.tld [x.x.x.x/NULL] error 110: Connection timed out first failed: 17-Jun-2023 13:35:15 last tried: 21-Jun-2023 13:44:15 next try at: 21-Jun-2023 21:44:15 past final cutoff time Transport: smtp2.recipientdomain.tld [y.y.y.y/NULL] error 110: Connection timed out first failed: 17-Jun-2023 13:35:15 last tried: 21-Jun-2023 13:44:15 next try at: 21-Jun-2023 21:44:15 past final cutoff time
This means that your email server was unable to contact the remote server for the specified domain.
This can be caused by many factors, such as ;
– Expired or non-existent domain name (or perhaps a typo error?) ;
– DNS issues (you have issues resolving the recipient domain or the recipient have DNS issues with their MX/A record being non-existent or pointing to the wrong record) ;
– The recipient SMTP server is down ;
– The traffic is being dropped/blocked at the source, in transit or at the destination (check to make sure you are not blocking traffic toward thhe remote mail service IP(s), test connectivity with telnet or NetCat).
Even though all of the above is possible, it is not uncommon sometimes that your sending server IP has been blocked at network level (firewall) on the other end, especially if you are using a shared service or public services such as Outlook.com, GMail, Yahoo, etc.).
You can easily test this with NetCat :
nc -vz x.x.x.x 25 nc -vz y.y.y.y 465