Emails to Gmail addresses

[#1]

Over the last 24 hours my emails from my Plusnet email address to any Gmail addresses have bounced back. Could this be connected with the recent Gmail outage or is it completely unrelated? Emails from an alternate Hotmail account get through to Gmail addresses. I use Thunderbird as my email client. Any help appreciated!

[#2]

On 15/12 two emails from my Plusnet account using Outlook were sent to GMail correspondents I contact regularly. Both bounced back "Mail delivery failed: returning message to sender". The 'explanation' for the rejection was just the receipients email address followed by a string of question marks, implying they don't exist. Yet shortly afterweards I received a message from one of them (GMail to Plusnet) with no problems. 

I've had longstanding problems with Plusnet>GMail, and they tend to be worse if I include a link or attachment - presumably GMail treats it as phishing. Yet a GMail address in France I write to regularly, almost always with links or attachments, never exhibits these problems.

It's reached the stage where I have to ask people with GMail addresses to send a mail receipt or short response to let me know emails have arrived. I would switch over to GMail for all my emails, except for the fact that I don't like their screen layout and means of filing messages. 

It's just another example of my favourite saying. "Technology: Love it or hate it, but never trust it!"

[#3]

There has been a host of issues with variable Gmail performance. We have seen Gmail rejecting emails sent to free accounts whilst accepting those sent to paid for accounts.

Similarly we’ve seen acceptance of emails sent using personal domain names via Plusnet ... but rejected if sent from the ‘an account’s naked’ account.plus.com domain name.

Gmail is something of a lottery.