Outgoing emails showing as 'Failed' from my phone

[.MGB]

On my phone (Galaxy A01) many of my outgoing emails do not send and show as 'Failed' in the outbox. These emails are to individual recipients and without attachments. When I send the same emails from my PC, they go through without problem. I have emailed the same recipients before without problem. The problem is intermittent but getting worse. I have tried sending them from my sky login at yahoo.co.uk via chrome and from the gmail app on the phone with the same result. Any suggestions as to why this happens please

[Mark39]

Is a failure reason given?

[.MGB]

Thanks for the reply. No, the email just has 'Failed' marked against it. The odd thing about gmail is that I can't see the out box, just messages at the top of the inbox saying that they have failed. . Also now a message appears when tapping the 'Junk E-mail'  icon saying 'There was a problem while syncing this folder. Please try again later' 

[.MGB]

Just realised I can see the outbox if I tap on 'Search emails' at the top.