Can not send emails on my MacBook Air when using the Comcast email service

sandip61 wrote:

I can send emails on other devices like iPad and iPhone and through the Xfinity website but cannot send through my MacBook Air

you can ref:

If you can't send emails in Mail on Mac

if you are having comcast specific email issue— contact there support.

If in doubt refer to the developers website; Support/Help/FAQ/known issues/compatibility /updates

Contact a third party vendor

Contact a third-party vendor - Apple Support

It is necessary to understand that in order for the email app to connect to the email provider's server, an exchange of information takes place. The user ID and password must then be accurately sent to the server within a specified period of time or a disconnect occurs. If there is any difference in what data is transmitted and what is being looked for, the connection will not occur. Therefore, something is not being sent correctly from your computer. You can open the email app and very carefully check  what you show for the user ID and password and correct it. Or you can delete the account from your email app and create a new one. That often solves the problem. If you choose to delete the account, be sure to first save any email messages to a local folder or they will be lost.