Recipients not receiving emails even though internet provider shows post in it's esnd folder. Started happening with no changes in configuratuin. Thanks

Sending emails from my laptop using Thunderbird I get the usual "connecting to...., sending to...,message sent" But recipients do not receive post even though my internet provider's sent folder contains the posts. Cox is my email provider and I can send posts from their WebMail site. There were no changes in configuration. I should note that I was using a "free" WiFi in Nicaragua. Also, I receive emails just fine. Cox says it's my email client but I don't see how it could be you guys isince Cox had had to receive the posts in order to put them in the sent folder. Thanks terrylf@cox.net