Problems sending email using Mac Mail program on desktop and iPad

Two to three weeks ago we suddenly became unable to send emails through comcast/xfinity when accessing our emails via Apple's Mail program on a desktop Mac and an iPad.  This is without us making any changes in passwords, email settings, etc.  We were able to receive emails without problems.  Surprisingly, we were able to both receive and SEND emails on our iPhones without any issues.  The assistants we have talked to at comcast/xfinity are unable to find a solution other than to access emails via the website.  They ignore the fact that the iPhone platform works just fine but using Apple's mail program on the desktop and iPad do not.  I found them to be less than helpful.  I saw at least one other comcast/xfinity user who is having this issue with their Apple computer.  Are there more Mac users out there with this same problem?  If so, have you found any solutions?