Apologies for the issues you're having with sending mail. This is actually the 2nd thread on this that I've come across today and I wonder if Gmail's recent update also included a change in their sending IP's? The error you're getting - VM311 auth denied 525 - indicates that this is a distinct possibility. I'll send you a PM in a moment (purple envelope icon, top right of page), for additional information and shall investigate further when I get your reply.
Sorry that you are also getting the VM311 error for sent mail. At the risk of repeating myself I'd like to refer you to my posted reply to David on here, message 4. Your information relating to this would be a huge aid to my investigation so please reply to the PM I've sent you (purple envelope icon, top right of page) and I'll look into this.
Thanks for getting back to me. Since I posted yesterday I have learned that there was an authentication issue reported approx a week ago which has since been resolved by amending our 'mail per IP' limits. This would coincide with your post but as I was off work at the time I was unaware of what had happened.
Anyhow glad to hear it's all working well for you now, thanks for the update
im getting this too and everybody is stumped, Virgin media cant help EE ( my phone provider cant find a problem nor can my phone manufacturer ( Oneplus) . Googling it though it doesnt seem uncommon im pulling my hair out, someone must know how to fix it