The IP address or host of the server your mail runs on is being blocked by the recipient server, even though everything has been fine for weeks if not months. The most common culprit in this case is a spam filtering system at the recipient server that blocks connections from the sending server.
One of the errors you may receive as a bounce when this happens is the ‘retry timeout exceeded’ or something like ‘all hosts for ‘domain’ have been failing for a long time (and retry time not reached).’ The solution in this case is always to contact the recipient and request them to have your server’s IP address or domain name whitelisted.
Finding out your server’s IP address is as simple as running a PING command aiming it at your domain name, so if your domain name is yourdomain.com:
$ ping yourdomain.com
PING yourdomain.com (192.168.1.1): 56 data bytes
The second line above reveals the IP address of the domain yourdomain.com.
Report the IP address of your domain (discovered by running the PING command) to the recipient or their network operator.