You will find on this page a series of seemingly unrelated tips in Thunderbird. It is the fact that they allow you to temporarily override Thunderbird's global options that binds them. It isn't as confusing as it sounds; please read on.
Depending on your preferences, emails are forwarded either inline (in the body of the forward email), or as attachments (the forwarded email appears as an attachment to a blank email).
Whether from Thunderbird's main window, or from an opened email window, use the Message > Forward as sub-menu to pre-select an inline or attached forward email.
All the options described below are available under the Options menu, a new email window. Most of the application-wide ("global") version of these options are covered under Composition Options in Thunderbird, which we will assume you have read.
To invoke the spell-checker while writing an email, go to Options > Check Spelling.
Using Options > Spell as You Type, you can toggle on or off Thunderbird's on-the-fly spell-check feature, regardless of global options set under Composition > Spelling.
Regardless of the global Thunderbird options you may have set under Composition > Addressing > Return Receipt, you can toggle the return receipt request setting on or off for the current email by using Options > Return Receipt.
You can override Thunderbird's character encoding settings for an individual email by going to Options > Character Encoding. This will not affect your Display > Fonts settings.
Regardless of global email formatting options, you can use the following formatting for a new email: Auto-Detect, which should accommodate your sender's format, Plain Text Only, Rich Text (HTML) Only, or Plain and Rich (HTML) Text. These choices are available under Options > Format.
Email priority is not a global options, and needs to be set for each email, (which makes sense). The default for Thunderbird and other email clients is "Normal", in other words, no priority information. Most email clients support 3 levels of priority: high, normal, and low. Thunderbird offers five: Highest, High, Normal, Low, and Lowest. Microsoft Outlook 2003, which supports three levels of priority, recognizes Thunderbird's High and Highest as High, which is good.
Thunderbird offers the option to digitally sign and/or encrypt an email. We will cover email security for Thunderbird in an upcoming tutorial. Stay tuned!