Mail server sync issues

For the last year or more I often get a notice that an email failed to send just after I hit send. They usually seem to go out in a few minutes  most of the time, so I had stopped even checking on them. This morning I noticed that for the last 2 weeks none of the devices I use were sending emails. The sent folder had about 8 emails from the last 2 weeks that all went out late last night over a few hours. I discovered this because I sent an email to my wife in the middle of the night, and it hadn't come through yet this morning. After refreshing the email again on the device I used to send the email last night it finally went through. I noticed one odd thing in this look at what's happening, and it's that the email sent times on my phone and tablet are different by a few hours in all of the last dozens or so emails. Isn't the outbox timestamp maintained by cox, and shouldn't that sync between my devises? 

If Cox can't explain or resolve this,  then I'm going to be expecting some big credits on my bill.