Bounced emails

What is going on with Runbox? I use RB 6. A few weeks ago I got an email from Runbox support stating there was an error in the code (or something like that) and that some incoming emails were bounced back to the sender. They also said the issue was fixed. Apparently, it’s not. I am still getting snail mail letters from my credit card companies that statement emails are being sent back as undeliverable. After a number of times this happens, they send a paper statement and charge me for it. I have checked the accuracy of the email address each time I get one of these paper letters from various senders, not just one specifically. So, what is going on, Runbox?