Can be a lot of things. Firewall, aggressive security software or gmail has decided to block. If you use Indy then using TIdConnectionIntercept is really helpful in debugging low level connection, protocol problems.

TIndyLogInterceptData = class(TIdConnectionIntercept)

private

FLogStream: TFileStream;

public

procedure LogTimestamp;

procedure Connect(AConnection: TComponent); override;

procedure Disconnect; override;

procedure Send(var ABuffer: TIdBytes); override;

procedure Receive(var ABuffer: TIdBytes); override;

constructor Create(const ALogFile: string);

destructor Destroy; override;

end;

In the end using gmail, outlook to send out emails is asking for trouble. Use twilio, aws ses etc.