First of all, I have to say the issue isn’t Post Office. I want to be clear on that, I’m not, and never have blamed Post Office for this.
A while back sign-ups to my mailing lists stopped working when Post Office was used for the signup. This only happened on one of my servers, I tested the others and it was working fine, so we figured it was something to do with the server, but we never got to the bottom of it. Joe was on hand to help, but in the end, I decided to find an alternative solution.
As we looked into it we did discover that thousands, literally thousands and thousands across about 20 or so sites, of spam sign-ups were getting past Post Office. I deleted all the spam sign-ups and went about my day.
Today I found a site I forgot to move from Post Office to the new solution. I tested Post Office and it’s now working perfectly again.
In the interim nothing to our knowledge has changed on the server, so we’re left wondering if the sheer level of spam sign-ups was the root.
I suspect we’ll never know, but for reference, if you experience a similar situation with Post Office first go check the lists for spam sign-ups and remove them. See if that fixes things.