Hi Susie, this may possibly have something to do with some configuration settings. Please follow the link I’ve provided to check the configurations for your specific email client. If this issue does not correct please contact our Migration team at 1-855-463-9333 for further support. Thank you
Linda from exede your link sadly didn't have any info to set up on an iPhone. I'm having same issue. I don't even get an alert saying, hey your mail isn't being sent. But sometimes if I go directly into outbox and hit send again if will send. Makes no sense.