Emails are being rejected today

[bluepennylady]

Out of the blue all email notifications are being returned by eBay to our inbox with the following message:

Your access to submit messages to this e-mail
    system has been rejected.

We have used the same management software for 20 years,  Everything worked fine on Friday.  Today we cannot notify any buyer of outgoing shipments or cannot respond to inquiries. All messages are being returned with the above rejection code. 

[participatrophy]

Blind guess is either a security token expired or some API call was deprecated. The developer is the best person to contact. What's the name of the management software @bluepennylady ? Is the company still around?

[bluepennylady]

I checked that first
I am unable to reply to buyer inquiries even using normal software such as Thunderbird.  I replied to messages on Friday afternoon just fine.  We have used Thunderbird for years.  
I just renewed tokens for the API a month ago.  That's all current. 
We are using eBay's masked email address for the buyer and our account related email address.
nothing has changed on this side That I can find

I am going to use my online email account and try to respond to a buyer inquiry.  My staff does not have access to my eBay account. WE use API so no one has to touch the account .  orders are pulled into our software, notifications sent to buyers and tracking uploaded. 

The software tech says all is good on our side. But I will check again.  

[bluepennylady]

eBay seems to have fixed the issued.  Allowing emails to be sent through the eBay system.  What a relief!