why do my Mail accounts keeping going offline since I switched to comcast?

Did you check with Comcast tech support and/or their website re. that setting? As long as it works, it's fine, but Comcast is a bit particular (that would be an understatement), so I wouldn't change settings unless instructed to do so. I'd probably "suggest" them to the tech, but let him/her okay them.

NOTE: Comcast told me CHANGES this yr 2013, are NOT allowing PORT 25 as it has easily been hacked into an dto use 587 as below pictured; I'm guessing, but if that combo list above 25,465, 587 tries each one, then possibly it even trying the 25 makes it stop working? They said they were stopping all connections that come thru 25.

I have set that "custom port" to also be 587, but still MAIL at different times seems to go offline, and I do NOT know why either! I can send and get mail on web browser at comcast sight, but can't use MAIL! I tried deleting and reinstalling the account also, to no avail. 😟

This is from an email I recieved from Comcast 11-8-2012:

Dear Valued XFINITY Internet Customer, Action Taken: On August 1, Comcast announced that we will no longer support the use of port 25 for sending email. Many older email clients (such as Outlook Express) default to using this port to send email. Please read this message to understand how this action may impact your ability to send email, and what you should do next. Comcast Webmail Users: If you use a web browser to access Xfinity Connect or other webmail service, this action will not affect your ability to send or receive e-mail. Email Program Users (Outlook Express, Outlook, MacMail, etc.): If you use an email program, this action will disable your program's ability to send email until you change your email program settings to send email on port 465. To protect your email security, click on the link for your current email software, then follow the step-by-step instructions to change your settings. Outlook Express Outlook 2003 Outlook 2007 Outlook 2010 Windows Mail Mail (Apple OS X) Don't see your email software? Then locate the preferences for your mail account in the software you use and provide the settings listed on this page. Prefer professional tech support? A highly trained, Tech Expert can remotely change your settings to secure your email. To find out about one-time service and fixes, please call 1-877-550-0793 or visitxfinity.com/SignatureSupport. If you are not using Xfinity Connect and use another email provider, please contact your provider for its recommended port settings. Most email providers offer an alternative to port 25 for sending email. If you have additional questions please visit our Help and Support site or our Customer Forums where many questions are discussed and answered by the Comcast community. Thank you for choosing Comcast! Sincerely, Comcast Customer Security Assurance

P.S. Here's the Comcast link dated 11-9-12 re no longer supporting smtp(Outgoing Mail Server) Port 25:

http://customer.comcast.com/help-and-support/internet/email-client-programs-with -xfinity-email/

I use Verizon DSL, and found that I needed to change the DNS from the default Verizon address IN THE MODEM ITSELF to someting like:

OpenDNS

The oldest of all services on this list, OpenDNS also supports billions of DNS queries per day via its servers operating on 208.67.222.222 and 208.67.220.220. In addition to its basic name resolution service, OpenDNS Home Solutions provides free Web site filtering of adult content (called FamilyShield) and URL spell correction with accompanying installed software. The company also sells various security products to businesses. Those looking for an alternative to Google products and a service that works well outside the U.S. also tend to like OpenDNS.

Google Public DNS

Google operates the world's largest public DNS service. Launched in December 2009, it supports many billions of DNS queries per day, much of the volume generated from clients outside the U.S. Google Public DNS utilizes servers at IP addresses 8.8.8.8 and 8.8.4.4. The company developed this service as one of its corporate initiatives to make the Internet more accessible and easier for everyone to use, making it a logical choice for users worldwide.

When I did this, the problem went away for good.

I have had same problem for many months. I tried 3 times to get phone support from Comcast but there was NO one who knew what to do; they just guessed and nothing worked. I am now "down" on my 2 Mac laptops. I tried what people suggested; when I re-entered my password that worked for ONE email send; otherwise, no luck. Why is it that Comcast can't fix this? !! On my new Air I have the imap as the incoming. Very frustrating.