Must admit I sometimes struggle with the same thing. It doesn't want to send sometimes, but most times it's fine.
Just checked and it's all okay (at this moment). I haven't made any setting changes on the phones beyond the original configuration, so unless it's the mail app in android changing and causing the fault I am at a loss too.
Guessing your using the Samsung Mail app? com.samsung.android.email.provider. Samsung Mail hasn't been updated since the middle of August.
I use the stock Gmail app. com.google.android.gm. Gail app was updated on the 8th October.
I do know that there was a problem for a little while where, in the gmail app at least, emails with attachments refused to go. But shrug, it's all working now.
Are you sending any attachments, if so what's the extension (bleh.txt, would be .txt). Virgin block some extension types, see here for the list.
---- I do not work for VM, but I would. It is just a Job. Most things I say I make up and sometimes it's useful, don't be mean if it's wrong. I would also make websites for them, because the job never seems to require the website to work.