I am struggling to send emails from writer because of Default Control problem
LibreOffice only supports email clients that allow MAPI protocol. GMail doesn’t. You can’t use GMail application with LibreOffice.
I assume you use Windows.
Protocol name is IMAP 
More precisely GMail is a mail service. Once you’ve configured your email agent UA, you can send your email through any “post office” (MTA) you have subscribed to.
Most common users read (and send) their emails with a web browser; this is probably what you mean “GMAIL”. A web browser is not an email user agent. It is just a viewer for an application residing in the provider server. This front-end is just used to display or enter text.
A mail user agent locally encodes message text and routing information according to standards (X.400 among others) and transfer that to MTAs on a port different from 80, 8080 (http) or 443 (https).
@ajlittoz: IMAP is a protocol of communication between the mail client and mail server. Simple MAPI (a subset of MAPI) is the protocol and API used by LibreOffice to communicate between LO and the mail client on Windows (of course, MAPI also can be used to communicate to mail server (Exchange), but that’s not what is used in LO).
@mikekaganski: Thanks for this info. I had already left LO’s orbit and was wandering in the Internet. Then MAPI is the link between LO and UA…
Please translate that for those of us who do not speak computereze. Surely there is a way to send a document from LibreOffice via gmail.
Please translate that for those of us who do not speak computereze
Don’t require some clarifications where none is possible. People are OK with “No GMO” labels, despite they generally have no idea what gene is, and can’t tell cytosine from thymine. They are OK when they are told that their gadgets need USB-C charger, although they usually couldn’t tell what USB abbreviation stands for, and can’t tell the specs of the type-C. So here there’s a technical requirement that mail client must support Simple MAPI to allow LibreOffice to communicate to it. You should consult your mail client manual to know if it supports it; there’s nothing we could do here, because we simply might not know every Simple MAPI compatible and incompatible client out there.