A "couldn't resolve host" message indicates that your webserver had a problem looking up the IP address of the destination server at the moment it needed to.
That can happen for a number of reasons:
A. Problems with your webserver:
- DNS service not responding properly ... overloaded, misconfigured, etc
- troubles connecting to external addresses
B. Problems with your hosting company's network
- DNS service on other DNS servers at your hosting company's server-network
- general internet connection problems
C. Problems with public DNS services
- perhaps there are some DNS replication problems happening
D. Problems at the destination server end
- perhaps the destination's servers are overloaded, running maintenance, or just having problems at that moment.