Worth checking BT mail settings I would think first and also check there are not service issues in case.. I am assuming you are a BT customer ...
IMAP
Incoming Mail Server: mail.btinternet.com
Port: 993 (this should be automatically populated if you tick the SSL Encryption)
SSL Encryption: Enabled (but not STARTTLS)
Username: your email address including the @btinternet or @btopenworld.com part
Password: your btinternet password
Root folder/path:
Folder separator: . (full stop)
Please do NOT enable "secure password authentication" (SPA). It's not needed when you use SSL encryption and won't work.
If your IMAP client supports it, we recommend you also set up the following folders:
Trash
Sent Items
Drafts
POP3
Server: mail.btinternet.com
Port: 995 (this should be automatically populated when you tick SSL Encryption)
SSL Encryption: Enabled (but not STARTTLS)
Username: your email address including the @btinternet or @btopenworld.com part
Password: your btinternet password
POP from folder: By default, POP will pull emails from your Inbox
SMTP
Outgoing Mail Server: mail.btinternet.com
Port: 465 (this may not be automatically populated on selection of SSL, so you'll need to check this)
SSL Encryption: Enabled (but not STARTTLS)
Authentication: PLAIN
Username: your email address including the @btinternet or @btopenworld.com part
Password: your btinternet password