Issue sending emails


I have a HP Lap top bought 2 weeks ago, running Windows 10.

I use BT Mail for reading and sending emails.

Over the last couple of days I have had an issue when sending emails either as a seperate item or a reply.

The screen reduces its brightness and the message sending emails, however it seems to last forever. After a while I cancel it and note that despite this my emails are showing in my sent folder.

Could someone please advise, many thanks