alternate e-mail address

Some questions:

1. Are you still an ATT uverse customer or using its email after cancelling uverse service (which is OK)?
2. Is your main account a legacy account (e.g., sbcglobal.net, pacbell.net, etc.)?
3. Are you using an email client app (e.g., Thundebird, Outlook, etc.) or the webmail?  If client app can you log into the webmail through your browser?
4. When was the last time you "re-verified" the accounts?  Can you still do that?
5. Are these sub-accounts or att/yahoo email send-only accounts?
6. Is your main account a merged att/yahoo account?

Currently and for the past two years I have been an AT&T Uverse customer, but was AT&T DSL customer when the alternate e-mail addresses were established and verified.  Problem is from my main account, an sbcglobal.net "legacy" account.  Problem of not being able to send from the alternate e-mail address occurs with Thunderbird client app, not from webmail.  Have never been able to use alternate e-mail address from webmail, only from Thunderbird.  It has been several years since I verified the alternate addresses, and now I see no facility within AT&T to do so.  Hope this helps arrive at a solution.

This happened to me as well, but only to one of my alternate email addresses.  I was not able to get any help from AT&T.

I don't have a solution here.  In the context of att/yahoo email I don't understand the concept of an "alternate" account other than sub-accounts and send-only accounts.  And I don't understand an att/yahoo account that can only be used through an email client app and not through the web interface where you either log into that account or use the Accounts icon in the webmail to select the desired account.

I have three alternate email addresses.  These allow me to send mail using my desktop mail client through the outbound.att.net SMTP server when using my system at home.  Two of the addresses are personal domains and the third is my work address.  Only one of my alternate email addresses (I think Yahoo called them aliases) is no longer working.  I haven't logged in to the AT&T system since before the Verison takeover of Yahoo.  They all transferred over to AT&T's server and worked fine until sometime last week.  The email clients on all of my devices are affected, or I would suspect some inadvertant configuration change on my part.

Seeing exactly the same thing here. The Yahoo Small Business status page shows it as a closed issue, but it obviously shouldn't be closed. I don't know how to contact them to tell them otherwise. Ideas?

https://help.smallbusiness.yahoo.net/s/system-status?tabset-99339=52964

This has happened to us as well and have gotten nothing from AT&T.  Before using U-verse/At&t internet, we were dial up customers with an email address from that company.  We had used that address for many years and wanted to keep it and have been doing so for over 1 year until about 2 weeks ago.  My inbound server was set to bring in that email account, then my outbound server was set to AT&T.  I can now send email, if I change my from email account to my att.net email address. Of course this is a pain, because I am juggling 2 email accounts.  The cost of my UVerse has gone through the roof and all they want to do is offer me DirecTv, no Thanks!!  Hope this helps, because I was thinking that it was my att.net account was the "mailbox failed" error, but it was the fact that AT&T did not like my alternate email!

Unfortunately no solution to this alternate e-mail problem has been forthcoming.  It looks like AT&T just took away the capability some of us were using for many years.