We have a small office network (7 users) using Thunderbird (IMAP) and Gmail. It has worked virtually trouble free for several years but suddenly several users are finding that sent emails never arrive. These emails do go into sent box and sometimes there is a :- 'Cannot connect to IMAP server' message GMail Webmail appears to wok perfectly on these accounts (though it doesn't get a great deal of use so I can.t be sure it is 100%) I have tried deleting user password and the re-entering when prompted and also deleting an account and re-installing it. OS Win7 Thunderbird 52.8.0 Incoming Server imap.gmail.com Port 993 Security SSL Authentification OAuth2 Outgoing server smtp.gmail.com Port 465 Security SSL Authentification OAuth2 Extensions:- Lightning 5.4.8 Bullguard 18.0 (Outgoing email scan OFF) I would be very grateful for any help on this problem