PN web mail still rejects valid email addresses

[#1]

Using webmail I cannot reply to some emails (sent via the spamgourmet service) because the web mail client rejects the email address.  I'm sure I reported this before but can't find the post. Please fix this.
And why on earth have you added CAPTCHAs?  I hate these since I can't read the damn things.

[#2]

Hi mapletree,
Can you tell me what message you get, what address are you replying to? I'd need as much info as possible to be able to investigate.
Jojo

[#3]

email address is like +xx+xxxxxxxx+aaaaaaaaa.xxxxxx#xxxx.com@spamgourmet.com
(I have replaced some words with "xxxx" & "aaaa"  for privacy.  But I see this error with all similar addresses).
The error message is:
"Message not sent. Server replied:
    Requested action not taken: mailbox unavailable
    550 restricted characters in address"
I can reply to these emails using a normal email client, just not via web-mail.