instead of gaining 230 MB space on my disk, the free space on it was decreased by 230 MB.

How did you determine this?

What will happen if I delete TB cache?

What does this mean? Empty the contents of the cache, or delete the cache folder in the file system? In case of the latter, why would you want to do this?

1. I determined this by comparison of the free space on my HDD before and after the operation 2. In TB Tools | Options| Advanced | Network & Disk Space tab: in Disk Space section, we have info "your cache is currently using 1227 MB of disk space". Next to it, there is the Clear Now option. In the next row one can see "override automatic cache management" and "Use up to .... MB of space for the cache". I meant to use Clear Now option or limit allowed space for the cache to some 512 MB because I am short of the free disk space on my HDD. My question is: Will I loose my e-mails, if I do this?

1. I determined this by comparison of the free space on my HDD before and after the operation

I believe you that you saw the free space on your disk decrease, however, I doubt that the folder you compacted increased in size. A more reliable way would be to compare the size of the Inbox file prior to and after the compacting.

Will I loose my e-mails, if I do this?

No, you can safely clear the cache, it will not impact your email messages. Having said that, it is always a good idea to have a current backup of the Thunderbird profile folder. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

1. I was able to see strange changes in the free disk space after compacting the TB folders before. I didn't react because of the accuracy problem involved: a) the setting "Compact all folders when it will save over 50 MB in total" and b) the fact that the free disk space of the HDDs in My Computer were displayed (in GB) with only one decimal digit. In such a case rounding of errors might also play negative role. In my present case, however, the freed disk space should be bigger and the described effects shouldn't happen. I don't suspect TB to 'swallow' somehow the free disk space while compacting TB folders. It seems to me that the free space gained when compacting folders could be marked for release at a later time, as it happens e.g. with some deletions in Windows. 2. Answer to Q2 sounds reassuring and I appreciate your hint to backup Profile, which I didn't take care of. Thanks a lot for your help.

In my present case, however, the freed disk space should be bigger and the described effects shouldn't happen.

There is a known issue with compacting. Thunderbird can state a way bigger number of disk space to be freed by compacting than what is actually freed. You really should check the size of the Inbox file to be compacted.

With this info about TB compacting issue, you convinced me that such a test is absolutely necessary. I'll do it during another big cleaning of my TB e-mails. Thanks for your help.