Is this an imap or pop mail account? Does the message get stored in 'Sent' folder? What is the error message ...word for word? What outgoing smtp server do you use?

1) Pop mail

2) No not stored in sent folder

3 & 4} "Sending of the message failed. The message could not be sent because the connection to Outgoing server (SMTP) smtp.comcast.net timed out. Try again."

Right click on pop mail account and select 'Settings' Scroll down on left side and select 'Outgoing server (SMTP)' Select the comcast outgoing server smtp in the list. Please post an image showing the settings.

Have you been travelling about recently, moving computer between different internet connections?

Do you have an Anti-Virus program which is scanning incoming and outgoing mail ? If yes, please perform a test, access AV settings and switch off scanning of incoming and outgoing mail and try to send an email. It is possible the scanning is slowing down the process and causing an issue with sending.

Please report back on result and state what AV product are you using?

There is no list. only one item on outgoing server... "Comcast.smpt.comcast.net (Default)"

I have not moved about at all. Laptop stays here at all times.

I am using Avast Free Anti-virus. Will try to access AV settings and let you know.

The issue has been sporadic lately...sometimes, when it won't send out, I close out of Thunderbird...reopen...and it will send. Other times, it does not work. Frustrating.