My gmail emails don't reach destination when using emclient but do then I use something else...

On my PC, I manage my gmail mail with eMClient.
Most of them reach the destination but to some addresses, sometime it does, sometimes it doesn’t .

For the same gmail account, if I send the very same message with google (on browser) or on my cell, it always work.

The message is always visible in the “sent folder” on eMClient.  If the message does not reach (I call the friend and ask),  it’s sometime visible on google gmail (on browser) and sometime not.

I can’t point out what’s different with the message that work and those who doesn’t.

Can you help me with this ?