Repeated message 'Thunderbird: The certificate for imap.shaw.ca ' keeps showing when try to read an e-mail. Yesterday a 'sent' e-mail did not register in my 'sent' box-checked it was received.I go into myww

Explain the problem I have already described!! - When I try to read an e-mail a 'box' appears saying 'Thunderbird: The certificate for imap.shaw.ca does not come from a trusted source'--What is this for?? _ Yesterday an e-mail I sent to several contacts showed as 'Sent' BUT did not register in my 'Sent' folder ----I had to phone England to check that they had received the e-mail, which they had. Today I was able to send e-mails & they were recorded in my 'sent' folder BUT the other problem still persists. Please advise if there is something that can be done to correct this problem. Thank you Phyllis M.