Is your db off-loaded? I suspect if db issues were the problem you'd see a lot more issues than notifications, but I just want to be thorough.
Are you using a mail relay? The more details about how you are sending mail, the more we will be able to help. To that end, can you PM me your Profile? You can download it by going to Admin > System Config > System Profile and click the ***Download Profile*** button towards the top. If for whatever reason you *cannot* download the profile, please put the output of View System Info (5.3.4+, Show Profile if older) in the thread (that will at least get us some info). This will give us access to many of the logs we would otherwise ask for individually. If security is a concern, you can unzip the profile take out what you like, and then zip it up again. We may end up needing something you remove, but we can ask for that specifically. The nagios.log is going to be the most important piece of the profile.
After you PM the profile, please update this thread. Updating this thread is the only way for it to show back up on our dashboard.
That blue button that says "Download Profile"...what happens if you press that? If it downloads a zip file, please attach it or PM it. If you PM, please make sure you update the post.
There is no need to change IP Address in Nagios database. I have changed IP Address then given hosts entry, then made changes in mail & SMS relay. Issue has been resolved.
Glad to hear it is resolved. I am going to lock the thread. Please feel free to post again if you have you another issue. Thank you for using the Nagios forums!