Inbox

[narphoenix]

The PM box holds only 40 messages. I've found that this can get really restrictive, especially when I can go through that number in a day or two. Is there any way to expand the inbox?

[Dina]

Post more!

[Priscellie]

The PM box holds only 40 messages. I've found that this can get really restrictive, especially when I can go through that number in a day or two. Is there any way to expand the inbox?

Nope, sorry.  Ask the folks you PM a lot for their email addresses or IM usernames.  PMs aren't intended for that kind of high-volume communication.

[Serack]

The PM box holds only 40 messages. I've found that this can get really restrictive, especially when I can go through that number in a day or two. Is there any way to expand the inbox?

The PM limit was a deliberate choice by the guy that owns the boards.  Here's his post on it from back in 2010 when he put the limit in place

Very shortly (some time today) I'm going to put in some hard limits on the number of PMs that people are allowed to keep on the system. Four of you have several thousand. A number have more than 100. I'm likely to set the limit at 40 or so. This isn't an email system!

Source

Around the time this was implimented the forums had to go through a pretty major transition because the growing popularity of Jim's works and therefore his official forums caused the forums to grow in bulk to the point that the forum's server was overtaxed.  This policy was one of many that brought the forums back down to a manageable size and greatly improved the overall stability of the system.