A recent update to a mail server SSL certificate has caused issues in lots of email clients.
An email provider I use recently updated their SSL certificate, as is good practice. However this seemed to cause unexpected effects in various mail clients. So far I've seen impacts in IOS Mail, Thunderbird and MacMail, other clients seem unaffected. The symptoms are your client suddenly stops downloading emails, for no apparent reason. The way round the problem is to turn off SSL if it's on, which seems ok for Thunderbird and IOS Mail. Mac Mail requires a few more tweaks including ensuring you've got the latest version running.