Why are emails being bounced as blocked for all Att email addresses

Hello @MaryDosterDRFFA ,

Some domains use data limiting to send only a certain amount of emails to certain clients from a single sender to limit data usage. The ones within the ATT/Yahoo sphere for these data limits include (but aren't limited to): Bellsouth, SBCglobal, ATT, Prodigy, and others managed by Yahoo (formerly by AT&T). This can be especially noticeable during election and holiday seasons when a higher-than-normal number of emails are being sent in general.

With that said, I would recommend the following:

1. Get your account self-authenticated. This makes your emails look more official, even when they're being sent through a 3rd party system (ours, in this case), which can overcome some of these systems' more stringent algorithms. For more info, please see our Community's Email Authentication FAQ. Since this can be pretty technical, it's recommended you work with your organization's IT or your domain host's support. 
2. Speak directly with our Delivery team for more specialized insight on your account's delivery. While "blocked" may be the official code the ATT (and related) servers sent back to us, there may be something more specific in that code our delivery teams can pull for your contacts. 

Ultimately, my previous experience when this has occurred for ATT-related addresses, is that it is temporary just due to this time of year. However, I would still suggest talking to the Delivery team to see if there’s anything (including self-auth) you can do to improve your delivery overall.

If this becomes a chronic issue for your delivery to these contacts, you may need to reach out to them from outside our system, and encourage them to provide alternative email addresses you can send to that don't use one of the domains listed earlier.