ATT Block 553 Error On Phplist Emails

We have several clients who use phplist for their opt-in lists. ATT and a few others are blocking emails. The main reason is because the email from the list (email@client.com) originates from comcast.net. ATT and others appear to block based upon that discrepancy.

Questions for anyone who works with phplist:

1) If the client simply sends from the comcast.net email (and not from email@client.com) and we allow this as a valid sending source in phplist, will this solve our problem?

2) We attempted to send via webmail, which would have trumped all issues, however because the clients are sending email which incorporates graphics in a template, webmail is a poor choice.

3) Is there another workaround that we are not seeing?