Who is the mail provider, and is the account POP or IMAP? It could be the messages are being copied to a folder that you haven't subscribed to, in the case of IMAP.

Mozilla Thunderbird is my email provider.

By 'mail provider' I mean who is the account provider, e.g. gmail, yahoo, BT etc.

Catcomm is my internet provider and Thunderbird is the email program that we use.

Internet provider is not always the same as mail provider. What comes after the @ in your email address? Under Tools/Account Settings, select Server Settings for the account and find Server Type - POP or IMAP?

We are using a POP Mail server. My internet & email provider is Catcomm, Thunderbird is my email program. My server name is "mail.moutonlong.com"

Can you confirm if the mail is actually being sent, i.e. they arrive at the recipients' end, but are just not being copied to Sent? Is this a recent issue, and was it working before with the same settings?

Yes, the recipients are receiving the emails, they are just not being saved into the sent file. It all happened when we switched over to IMAP from POP setting.

dbhickman said

Yes, the recipients are receiving the emails, they are just not being saved into the sent file. It all happened when we switched over to IMAP from POP setting.

I thought you said earlier the account was POP? When you switched from POP to IMAP, or vice versa, did you add the account with the different protocol, or simply change the server settings? When you make this kind of switch, it's not sufficient to just change the existing settings. This may be the reason the Sent mail is 'missing'.

http://kb.mozillazine.org/Convert_a_POP_account_to_a_IMAP_account