Moodle uses a method to generate e-mail addresses for replying.

It seems, Moodle is actually following the relaxed RFC's to support local-parts that have all kinds of ASCII characters, however systems like Exim allow administrators to block certain characters in the local-parts of the domain.

While it's technically allowed to use these characters, it will not function in 100% of the cases, as some admins don't know certain local-part characters to be blocked.

For example, with Exim and the Direct Admin control panel, the following is blocked by default:

Any local-part starting with a . character, or containing the characters @ % ! / |

For example, the default Debian and Ubuntu install of Exim, will not allow the following characters (in regex format):

CHECK_RCPT_LOCAL_LOCALPARTS = ^[.] : ^.*@%!/

While both cases are still restricted to Exim, this is still quite a userbase, as Exim is the default MTA for Direct Admin / WHM / cPanel.

Do note, that according to RFC 3696 using these chars is allowed, however, for example the ! char could still be used in UUCP bang path, so it could be logical to just skip those chars.

This was suggested earlier in MDL-52109, however that report also asked for - and + to be removed, which are quite acceptable these days.

If deemed this won't be fixed, do please update the documentation, so the few users actually using this feature can find the possible reason for this to be failing.