Sending of the message failed. The message could not be sent because the connection to Outgoing server (SMTP) smtp.googlemail.com timed out. Try again

I am on an iMac and have been happy with Thunderbird for 4 years. All changed 3 days ago when I tried to send a Jpeg attachment and then emails without attachments. I receive emails fine, sending is now the problem. There is a long delay connecting to server (Gmail), then I receive "Sending of the message failed. The message could not be sent because the connection to Outgoing server (SMTP) smtp.googlemail.com timed out. Try again" . I switched the Firewall on and off without effect. I tried changing Account Port nos from 465 to 586 without effect. Now back on 465, normal password. I can occasionally send mail, but it takes a long time and often fails with the "Sending of the message failed. The message could not be sent because the connection to Outgoing server (SMTP) smtp.googlemail.com timed out. Try again" It is very frustrating I have switched to Apple Mail today, which works fine sending mail and attachments.